# Math Help - binomial theorem factorial

1. ## 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

2. 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 .