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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