# summation help

• Feb 16th 2009, 02:57 PM
mathh18
summation help
which of the following express 1+2+4+8+16+32 in sigma notation?
a.) 2^(k-1) from k=1 to 6
b.) 2^k from k=0 to 5
c.) 2^(k+1) from k= -1 to 4

I don't understand, isn't it all of them??

Also, use formulas to evaluate:
(1-k)*(2-k) from k=0 to k=n

I'm confused on this problem because I only know formulas for summations from k=1 to k=n. Anyone know how to do this one?
Thanks!!
• Feb 16th 2009, 03:02 PM
wytiaz
yes, choices a, b, and c are all the same and are each expressions of the series
• Feb 16th 2009, 03:03 PM
Jhevon
Quote:

Originally Posted by mathh18
which of the following express 1+2+4+8+16+32 in sigma notation?
a.) 2^(k-1) from k=1 to 6
b.) 2^k from k=0 to 5
c.) 2^(k+1) from k= -1 to 4

I don't understand, isn't it all of them??

yes it is. but perhaps you are to conform to a certain definition. for instance, they may expect you to start from k = 0, or k = 1. starting at k = -1 seems awkward at this level

Quote:

Also, use formulas to evaluate:
(1-k)*(2-k) from k=0 to k=n

I'm confused on this problem because I only know formulas for summations from k=1 to k=n. Anyone know how to do this one?
Thanks!!
Hint: (1) $\sum_{k = 0}^nf(k) = f(0) + \sum_{k = 1}^nf(k)$

(2) you have $\sum_{k = 0}^n (2 - 3k + k^2)$
• Feb 16th 2009, 03:08 PM
mathh18
thank you!