# summation evaluation

• Nov 4th 2008, 09:11 AM
pandakrap
summation evaluation
could I get some help with evaluating this sum.
• Nov 4th 2008, 09:36 AM
PaulRS
Note that your sum is equal to: $\displaystyle \sum\limits_{k = 0}^{2008}{\left( { - 1} \right)^k \cdot \binom{2009}{k}\cdot{\binom{2008}{2008-k}}}$

And that corresponds to the coefficient of $\displaystyle x^{2008}$ in the product: $\displaystyle \left( {1 - x} \right)^{2009} \cdot \left( {1 + x} \right)^{2008}$ ( use the Binomial Theorem)

On the other hand: $\displaystyle \left( {1 - x} \right)^{2009} \cdot \left( {1 + x} \right)^{2008} = \left( {1 - x} \right) \cdot \left( {1 - x^2 } \right)^{2008}$

By the Binomial Theorem:$\displaystyle \left( {1 - x} \right) \cdot \left( {1 - x^2 } \right)^{2008} = \left( {1 - x} \right) \cdot \sum\limits_{k = 0}^{2008} {\binom{2008}{k} \cdot \left( { - x^2 } \right)^k }$

So we recognise the coefficient of $\displaystyle x^{2008}$ as $\displaystyle \binom{2008}{1004}$
• Nov 4th 2008, 10:18 AM
pandakrap
:)
thanks a bunch