# summations

• September 27th 2010, 02:44 AM
collegestudent321
summations
Hello,
I am having difficulty solving a summation because both the upper bound is a different variable than the lower bound. The summation is as follows:

2n
E (i) = ?
i=5

The "E" represents the sigma. I had considered the following:
5(0)n + 5(1)n + 5(2)n = 15n
but I honestly have no idea if that is correct. Please let me know if I am on the right track. Thank you!
• September 27th 2010, 02:52 AM
Plato
• September 27th 2010, 03:28 AM
collegestudent321
That makes perfect sense. Thank you very much!
• September 27th 2010, 03:44 AM
collegestudent321
Ok, so how about one more...

n-7
E 2i + 7
i=0
• September 27th 2010, 04:08 AM
Plato
Quote:

Originally Posted by collegestudent321
Ok, so how about one more...
n-7
E 2i + 7
i=0

$\sum\limits_{i = 0}^{n - 7} {2i + 7} = 2\sum\limits_{i = 0}^{n - 7} {i + } \sum\limits_{i = 0}^{n - 7} 7 = 2\left( {\frac{{\left( {n - 7} \right)\left( {n - 6} \right)}}
{2}} \right) + 7\left( {n - 6} \right)$

Note that as $i=0,\cdots n-7$ gives $n-6$ terms.
• September 27th 2010, 04:23 AM
collegestudent321
Ok, the (n-6) terms part had me confused for a little but i got it now. Thank you once again for your help!