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Appendix C C-111<br />

PROJECT<br />

More on Sums<br />

In this project we develop some algebra for simplifying sums. First, here are<br />

four important properties of summation.<br />

Properties of Summation<br />

Let a 1 , a 2 , . . . a n , b 1 , b 2 , . . . b n , and c be real numbers. Then<br />

n<br />

n<br />

n<br />

n<br />

n<br />

n<br />

1. a (a k b k ) a a k a b k 2. a (a k b k ) a a k a<br />

k1<br />

k1 k1<br />

k1<br />

k1<br />

n<br />

n<br />

n<br />

3. a ca k c a a k<br />

4. a c nc<br />

Proofs<br />

1. To prove Property 1 we write out the sum term-by-term then use the associative<br />

and commutative properties of addition to remove parentheses then<br />

rearrange and regroup terms.<br />

n<br />

a (a k b k ) (a 1 b 1 ) (a 2 b 2 ) p (a n b n )<br />

k1<br />

k1<br />

Definition of<br />

summation<br />

notation<br />

Remove<br />

parentheses<br />

Commutative<br />

and associative<br />

properties of<br />

addition<br />

Summation<br />

notation<br />

2. The proof of Property 2 is similar to that for Property 1. We leave it for you<br />

as Exercise 1.<br />

3. Property 3 is a statement of the distributive property or, equivalently, factoring<br />

out a common factor from a sum of terms.<br />

n<br />

a ca k ca 1 ca 2 p ca n<br />

k1<br />

k1<br />

a 1 b 1 a 2 b 2 p a n b n<br />

(a 1 a 2 p a n ) (b 1 b 2 p b n )<br />

<br />

c(a 1 a 2 p a n )<br />

n<br />

c a<br />

a k<br />

k1<br />

n<br />

a a k a<br />

k1<br />

Summation notation<br />

Factor out a common factor<br />

Summation notation<br />

4. Property 4 is tricky. The summation notation says that there are n terms and<br />

each term is the same number c:<br />

n<br />

k1<br />

n<br />

b k<br />

k1<br />

a c c c p c<br />

k1<br />

n terms<br />

nc<br />

∂<br />

b k<br />

k1

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