# binomial theorem factorial

• June 9th 2009, 01:54 AM
joey1
binomial theorem factorial
the first 3 terms in the expansion of (1 + ax) to the power n are 1+4x+7x to the power 2 ,find n and a
• June 9th 2009, 04:08 AM
Quote:

Originally Posted by joey1
the first 3 terms in the expansion of (1 + ax) to the power n are 1+4x+7x to the power 2 ,find n and a

$
(1+ax)^n=1+nax+\frac{n(n-1)}{2!}a^2x^2+...
$

$nax=4x$, $na=4$ ---1

$\frac{n(n-1)}{2!}a^2=7$ ---2

Solve the simultaneous equation then .