1. ## binominal coefficients

need help on this one.
used newton's binominal and its derivative but couldnot get the result.

2. Originally Posted by baz
need help on this one.
used newton's binominal and its derivative but couldnot get the result.
$(1+x)^n = \displaystyle \sum_{k=0}^{k=n} \binom{n}{k}x^k$

differentiate the equation on both the sides:

$n(1+x)^{n-1} = \displaystyle \sum_{k=0}^{k=n} k\binom{n}{k}x^{k-1}$

Multiply by $x$ on both sides:

$nx(1+x)^{n-1} = \displaystyle \sum_{k=0}^{k=n} k\binom{n}{k}x^{k}$

Now substitute $x = -1$ to get your answer