Book of Proof - Amazon S3

78 Counting
coefficients 1 2 1. Similarly (x+ y) 3 = 1x 3 +3x 2 y+3xy 2 +1y 3 , and Row 3 lists
the coefficients 1 3 3 1. Pascal’s triangle is shown on the left of Figure 3.3,
and immediately to the right of each Row n is listed the expansion of
(x + y) n . In every case (at least as far as you care to check) the numbers in
Row n match up with the coefficients of (x + y) n .
In fact this turns out to be true for every n. This result is known as
the binomial theorem, and it is worth mentioning here. It tells how to
raise a binomial x + y to a non-negative integer power n.
Theorem 3.1 (Binomial Theorem) If n is a non-negative integer, then
(x + y) n = ( n) 0 x n + ( n) 1 x n−1 y + ( n) 2 x n−2 y 2 + ( n) 3 x n−3 y 3 + ··· + ( n
)
n−1 xy n−1 + ( n
n)
y n .
For now we will be content to accept the binomial theorem without
proof. (You will be asked to prove it in an exercise in Chapter 10.) But
until then you may find it useful from time to time. For instance, you
can apply it if you ever need to expand an expression such as (x + y) 7 . To
do this, look at Row 7 of Pascal’s triangle in Figure 3.2 and apply the
binomial Theorem to get
(x + y) 7 = x 7 + 7x 6 y + 21x 5 y 2 + 35x 4 y 3 + 35x 3 y 4 + 21x 2 y 5 + 7xy 6 + y 7 .
Exercises for Section 3.4
1. Write out Row 11 of Pascal’s triangle.
2. Use the binomial theorem to find the coefficient of x 8 y 5 in (x + y) 13 .
3. Use the binomial theorem to find the coefficient of x 8 in (x + 2) 13 .
4. Use the binomial theorem to find the coefficient of x 6 y 3 in (3x − 2y) 9 .
5. Use the binomial theorem to show ∑ n n
k=0(
k)
= 2 n .
6. Use Definition 3.2 (page 72) and Fact 1.3 (page 12) to show ∑ n n
k=0(
k)
= 2 n .
7. Use the binomial theorem to show ∑ n
k=0 3k( n
k)
= 4 n .
8. Use Fact 3.3 (page 74) to derive Equation 3.2 (page 76).
9. Use the binomial theorem to show ( n
(
0)
−
n
(
1)
+
n
(
2)
−
n
(
3)
+
n
)
4 − ··· + (−1)
n ( n
n)
= 0.
10. Show that the formula k ( n) (
k = n n−1
k−1) is true for all integers n, k with 0 ≤ k ≤ n.
11. Use the binomial theorem to show 9 n = ∑ n
k=0 (−1)k( n
k)
10 n−k .
12. Show that ( n k
(
k)(
m)
=
n
)( n−m
m k−m) .
13. Show that ( n
(
3)
=
2
(
2)
+
3
(
2)
+
4
(
2)
+
5
) (
2 + ··· + n−1
)
2 .