A First Course in Linear Algebra, 2017a

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134 Determinants The cofactor matrix is and so the inverse is ⎡ 1 ⎣ e t ⎡ C (t)= ⎣ 1 0 0 0 e t cost e t sint 0 −e t sint e t cost 1 0 0 0 e t cost e t sint 0 −e t sint e t cost ⎤ ⎦ T ⎡ = ⎣ ⎤ ⎦ e −t 0 0 0 cost −sint 0 sint cost ⎤ ⎦ ♠ 3.2.2 Cramer’s Rule Another context in which the formula given in Theorem 3.40 is important is Cramer’s Rule. Recall that we can represent a system of linear equations in the form AX = B, where the solutions to this system are given by X. Cramer’s Rule gives a formula for the solutions X in the special case that A is a square invertible matrix. Note this rule does not apply if you have a system of equations in which there is a different number of equations than variables (in other words, when A is not square), or when A is not invertible. Suppose we have a system of equations given by AX = B, and we want to find solutions X which satisfy this system. Then recall that if A −1 exists, AX = B A −1 (AX) = A −1 B ( A −1 A ) X = A −1 B IX = A −1 B X = A −1 B Hence, the solutions X to the system are given by X = A −1 B. Since we assume that A −1 exists, we can use the formula for A −1 given above. Substituting this formula into the equation for X,wehave X = A −1 B = 1 det(A) adj(A)B Let x i be the i th entry of X and b j be the j th entry of B. Then this equation becomes x i = n [ ] −1 ∑ aij b j = j=1 n ∑ j=1 1 det(A) adj(A) ij b j where adj(A) ij is the ij th entry of adj(A). By the formula for the expansion of a determinant along a column, ⎡ ⎤ x i = 1 ∗ ··· b 1 ··· ∗ det(A) det ⎢ ⎥ ⎣ . . . ⎦ ∗ ··· b n ··· ∗
3.2. Applications of the Determinant 135 where here the i th column of A is replaced with the column vector [b 1 ····,b n ] T . The determinant of this modified matrix is taken and divided by det(A). This formula is known as Cramer’s rule. We formally define this method now. Procedure 3.44: Using Cramer’s Rule Suppose A is an n × n invertible matrix and we wish to solve the system AX = B for X = [x 1 ,···,x n ] T . Then Cramer’s rule says x i = det(A i) det(A) where A i is the matrix obtained by replacing the i th column of A with the column matrix ⎡ ⎤ b 1 ⎢ B = . ⎥ ⎣ .. ⎦ b n We illustrate this procedure in the following example. Example 3.45: Using Cramer’s Rule Find x,y,z if ⎡ ⎣ 1 2 1 3 2 1 2 −3 2 ⎤⎡ ⎦⎣ x y z ⎤ ⎡ ⎦ = ⎣ 1 2 3 ⎤ ⎦ Solution. We will use method outlined in Procedure 3.44 to find the values for x,y,z which give the solution to this system. Let ⎡ ⎤ 1 B = ⎣ 2 ⎦ 3 In order to find x, we calculate x = det(A 1) det(A) where A 1 is the matrix obtained from replacing the first column of A with B. Hence, A 1 is given by ⎡ ⎤ 1 2 1 A 1 = ⎣ 2 2 1 ⎦ 3 −3 2
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with Open Texts Linear A Algebra wi
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A First Course in Linear Algebra an
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A First Course in Linear Algebra an
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iv Table of Contents 3 Determinants
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vi Table of Contents 7.3.5 The Matr
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Chapter 1 Systems of Equations 1.1
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1.1. Systems of Equations, Geometry
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1.2. Systems of Equations, Algebrai
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54 Matrices Consider the following
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56 Matrices Solution. Notice that b
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58 Matrices Proposition 2.10: Prope
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60 Matrices on the left by a vector
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62 Matrices will satisfy the equati
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64 Matrices Solution. First check i
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66 Matrices Solution. First check i
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68 Matrices Proposition 2.25: Prope
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70 Matrices Definition 2.29: Symmet
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72 Matrices Theorem 2.34: Uniquenes
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74 Matrices Let’s solve the first
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76 Matrices Notice that the left ha
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78 Matrices What if the right side,
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80 Matrices First, consider the fol
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82 Matrices You can see that B is t
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Page 100 and 101: 86 Matrices First: ⎡ ⎣ 0 1 0 1
Page 102 and 103: 88 Matrices Proof. Suppose that A a
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Page 106 and 107: 92 Matrices (h) DE ⎡ Exercise 2.1
Page 108 and 109: 94 Matrices (a) (A − B) 2 = A 2
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Page 112 and 113: 98 Matrices (a) Find the elementary
Page 114 and 115: 100 Matrices One way to find the LU
Page 116 and 117: 102 Matrices which yields very quic
Page 118 and 119: 104 Matrices By Lemma 2.70, this im
Page 120 and 121: 106 Matrices Exercise 2.2.12 Find t
Page 122 and 123: 108 Determinants The 2×2 determina
Page 124 and 125: 110 Determinants Definition 3.7: Th
Page 126 and 127: 112 Determinants The following prov
Page 128 and 129: 114 Determinants 3.1.3 Properties o
Page 130 and 131: 116 Determinants Example 3.23: Mult
Page 132 and 133: 118 Determinants Solution. Consider
Page 134 and 135: 120 Determinants = ∑a 1,l (cof(B)
Page 136 and 137: 122 Determinants Theorem 3.35: Let
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Page 140 and 141: 126 Determinants (a) minor(A) 11 (b
Page 142 and 143: 128 Determinants Exercise 3.1.13 An
Page 144 and 145: 130 Determinants 3.2.1 A Formula fo
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Page 150 and 151: 136 Determinants Therefore, ∣ ∣
Page 152 and 153: 138 Determinants After row operatio
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Page 156 and 157: 142 Determinants Does there exist a
Page 158 and 159: 144 R n Hence, every element in R 2
Page 160 and 161: 146 R n components are equal. Preci
Page 162 and 163: 148 R n Theorem 4.6: Properties of
Page 164 and 165: 150 R n 4.3 Geometric Meaning of Ve
Page 166 and 167: 152 R n 4.4 Length of a Vector Outc
Page 168 and 169: 154 R n Solution. We will use the f
Page 170 and 171: 156 R n = √ 1 + 9 + 16 = √ 26 I
Page 172 and 173: 158 R n ⃗u 2⃗v ⃗u + 2⃗v ⃗
Page 174 and 175: 160 R n ⎡ Let ⃗q = ⎣ Then, x
Page 176 and 177: 162 R n Let t = x−2 y−1 3 ,t =
Page 178 and 179: 164 R n Example 4.26: Compute a Dot
Page 180 and 181: 166 R n Notice that this proof was
Page 182 and 183: 168 R n Therefore, the cosine of th
Page 184 and 185: 170 R n 4.7.3 Projections In some a
Page 186 and 187: 172 R n We will conclude this secti
Page 188 and 189: 174 R n Exercise 4.7.11 Find proj
Page 190 and 191: 176 R n Solution. By Definition 4.4
Page 192 and 193: 178 R n From the above scalar equat
Page 194 and 195: 180 R n Definition 4.48: Geometric
Page 196 and 197: 182 R n We will now look at an exam
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184 R n 4.9.1 The Box Product Recal
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186 R n = a ∣ e h ⎡ = det⎣ f
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188 R n 4.10 Spanning, Linear Indep
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190 R n 4.10.2 Linearly Independent
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192 R n Theorem 4.66: Linear Indepe
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194 R n Therefore we can write ⎡
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196 R n Now suppose that⃗u ∉ sp
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198 R n A subspace is simply a set
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200 R n Definition 4.80: Basis of a
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202 R n ⃗u 1 = Furthermore, ⎡
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204 R n ⎡ ⎢ ⎣ 1 2 1 1 3 0 1 3
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206 R n Solution. An easy way to do
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208 R n Since {⃗r 1 ,..., p⃗r j
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210 R n Example 4.101: Rank, Column
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212 R n Definition 4.106: Image of
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214 R n Example 4.110: Null Space o
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216 R n Theorem 4.114: Let A be an
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218 R n Exercise 4.10.7 Here are so
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220 R n Exercise 4.10.17 Are the fo
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222 R n Thse vectors can’t possib
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224 R n ⎧⎡ ⎪⎨ Exercise 4.10
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226 R n Exercise 4.10.52 Consider t
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228 R n Exercise 4.10.68 Find the r
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230 R n Solution. We already verifi
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232 R n Thus to find a correspondin
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234 R n That is: We readily compute
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236 R n We will say that the column
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238 R n To show that {⃗v 1 ,··
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240 R n is an orthogonal basis of U
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242 R n = = = ⎡ ⎣ ⎡ ⎣ ⎡
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244 R n In order to find W ⊥ , we
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246 R n Now, we need to write ⃗y
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248 R n Theorem 4.150: Existence of
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250 R n Solution. First, consider w
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252 R n −1 1 2 3 4 5 6 y 4 2 x Re
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254 R n Exercise 4.11.7 For U an or
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256 R n and here is a point: (1,1,2
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258 R n What does addition of vecto
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260 R n 4 3 First we want to know t
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262 R n = −58 Newton meters Note
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264 R n Exercise 4.12.19 A girl dra
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266 Linear Transformations numerica
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268 Linear Transformations Exercise
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270 Linear Transformations In this
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272 Linear Transformations ⎡ ⎣
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274 Linear Transformations ⎡ T
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276 Linear Transformations ⎡ (b)
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278 Linear Transformations The nece
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280 Linear Transformations Then, co
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282 Linear Transformations More gen
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284 Linear Transformations Solution
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288 Linear Transformations Definiti
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290 Linear Transformations This tel
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292 Linear Transformations Example
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294 Linear Transformations 2. T is
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296 Linear Transformations Proposit
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298 Linear Transformations Proof. S
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300 Linear Transformations You can
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304 Linear Transformations for exam
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306 Linear Transformations Example
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308 Linear Transformations Since {T
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310 Linear Transformations Find a b
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312 Linear Transformations Theorem
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314 Linear Transformations and one
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316 Linear Transformations Hence
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318 Linear Transformations system.
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320 Linear Transformations Solution
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324 Complex Numbers Theorem 6.1: Pr
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326 Complex Numbers Example 6.5: Co
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328 Complex Numbers Definition 6.10
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330 Complex Numbers Exercise 6.1.5
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332 Complex Numbers Example 6.14: P
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334 Complex Numbers This requires r
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336 Complex Numbers Therefore, x 3
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338 Complex Numbers Consider the fo
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Chapter 7 Spectral Theory 7.1 Eigen
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7.1. Eigenvalues and Eigenvectors o
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7.2. Diagonalization 355 One eigenv
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7.2. Diagonalization 357 In words,
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7.2. Diagonalization 359 Conversely
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7.2. Diagonalization 361 Theorem 7.
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7.2. Diagonalization 363 7.2.3 Comp
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7.2. Diagonalization 365 Exercise 7
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7.3. Applications of Spectral Theor
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7.4. Orthogonality 395 [ x(0) y(0)
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7.4. Orthogonality 397 Solution. Fi
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7.4. Orthogonality 399 Therefore an
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7.4. Orthogonality 401 In the above
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7.4. Orthogonality 403 Theorem 7.62
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7.4. Orthogonality 405 Theorem 7.66
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7.4. Orthogonality 407 λ 1 = 16: s
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7.4. Orthogonality 409 [ ] [ ] AV1
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7.4. Orthogonality 411 = [ 4 0 0 0
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7.4. Orthogonality 413 by the assum
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7.4. Orthogonality 415 operations)
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7.4. Orthogonality 417 Notice that
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7.4. Orthogonality 419 insignifican
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7.4. Orthogonality 421 Usually it w
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7.4. Orthogonality 423 We now turn
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7.4. Orthogonality 425 Solution. No
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7.4. Orthogonality 427 = 2y 2 1 + 8
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7.4. Orthogonality 429 ⎡ A = ⎢
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7.4. Orthogonality 431 Exercise 7.4
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Chapter 8 Some Curvilinear Coordina
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8.1. Polar Coordinates and Polar Gr
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8.2. Spherical and Cylindrical Coor
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Chapter 9 Vector Spaces 9.1 Algebra
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9.1. Algebraic Considerations 451 E
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9.1. Algebraic Considerations 453 H
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9.1. Algebraic Considerations 455
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9.1. Algebraic Considerations 457 =
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9.2. Spanning Sets 465 Show that, w
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9.2. Spanning Sets 467 Clearlynoval
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9.3. Linear Independence 469 9.3 Li
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9.3. Linear Independence 471 Soluti
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9.3. Linear Independence 473 Exerci
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9.3. Linear Independence 475 Exerci
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9.4. Subspaces and Basis 477 (b) {
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9.4. Subspaces and Basis 479 1. U
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9.4. Subspaces and Basis 481 It fol
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9.4. Subspaces and Basis 483 But th
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9.4. Subspaces and Basis 485 Then {
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9.4. Subspaces and Basis 487 Then f
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9.4. Subspaces and Basis 489 Theref
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9.4. Subspaces and Basis 491 The re
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9.6. Linear Transformations 493 2.
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9.6. Linear Transformations 495 2.
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9.6. Linear Transformations 497 The
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9.7. Isomorphisms 499 9.7 Isomorphi
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9.7. Isomorphisms 501 Example 9.66:
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9.7. Isomorphisms 503 Example 9.71:
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9.7. Isomorphisms 505 Since T is on
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9.7. Isomorphisms 507 for ∑ n i=1
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9.7. Isomorphisms 509 Exercises Exe
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9.7. Isomorphisms 511 for example.
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9.8. The Kernel And Image Of A Line
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9.8. The Kernel And Image Of A Line
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9.9. The Matrix of a Linear Transfo
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Appendix A Some Prerequisite Topics
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A.2. Well Ordering and Induction 53
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A.2. Well Ordering and Induction 53
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Appendix B Selected Exercise Answer
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537 1.2.33 Solution is: [x = 2 −
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539 2.1.7 0A =(0 + 0)A = 0A + 0A.No
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541 [ 2.1.18 Let A = 2.1.20 Let A =
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543 2.1.40 2.1.41 ⎡ ⎣ ⎡ ⎣ 1
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545 2.2.10 An LU factorization of t
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547 3.1.14 If the determinant is no
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549 ⎡ 3.2.2 det⎣ 1 2 0 0 2 1 3
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551 3.2.16 This follows because det
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553 4.7.16 ( ) ( ) ⃗u •⃗v ⃗
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555 It follows that the expression
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557 4.11.6 (a) Orthogonal. (b) Symm
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559 4.11.14 Then a solution is ⎡
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561 4.12.23 ⎡ ⎢ ⃗F 1 • ⎣
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563 5.2.13 ⎡ 1 ⎣ 10 1 0 3 0 0 0
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565 Now to write in terms of (a,b),
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567 ⎡ 5.9.7 Solution is: ⎣ −
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569 6.1.6 If p(z)=0, then you have
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571 7.1.2 Say AX = λX.ThencAX = c
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573 7.3.1 First we write A = PDP
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575 7.3.19 The solution is [ e At 8
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577 7.4.12 The eigenvectors and eig
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579 9.3.30 No. They can’t be. 9.3
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581 9.8.5 We can easily see that di
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Index ∩, 529 ∪, 529 \, 529 row-
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INDEX 585 null space, 211 orthogona
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A First Course in Linear Algebra, 2017a

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