(sorry i dont know how to do the summation symbol)

double summation, j=1 to 4 and k=2 to 8 (j*k)

is there a specific formula you are supposed to use?

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- Apr 26th 2009, 07:32 PMjlt1209double summation
(sorry i dont know how to do the summation symbol)

double summation, j=1 to 4 and k=2 to 8 (j*k)

is there a specific formula you are supposed to use? - Apr 28th 2009, 02:16 AMMoo
Hello,

$\displaystyle \sum_{j=1}^4 \sum_{k=2}^8 jk=\sum_{j=1}^4 \left( \sum_{k=2}^8 jk\right)$

In the second summation, since j doesn't depend on k, you can get it out from the sum :

$\displaystyle =\sum_{j=1}^4 \left(k \sum_{j=1}^8 j\right)$

You can calculate the sum $\displaystyle \sum_{j=1}^8 j$, by using this known formula : $\displaystyle \sum_{k=1}^n k=\frac{n(n+1)}{2}$

And then, you're left with only one sum, that should be easy to compute ;) - Apr 28th 2009, 09:15 PMmatheagle
Typo alert.....

$\displaystyle \sum_{j=1}^4 \sum_{k=2}^8 jk=\sum_{j=1}^4 \left( \sum_{k=2}^8 jk\right)$

In the second summation, since j doesn't depend on k, you can get it out from the sum :

$\displaystyle =\left(\sum_{j=1}^4 j\right)\left(\sum_{k=2}^8 k\right)$

or

$\displaystyle =\left(\sum_{k=2}^8 k\right)\left(\sum_{j=1}^4 j\right)$