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
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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 $\displaystyle (1+ax)^n=1+nax+\frac{n(n-1)}{2!}a^2x^2+... $ $\displaystyle nax=4x $, $\displaystyle na=4$ ---1 $\displaystyle \frac{n(n-1)}{2!}a^2=7$ ---2 Solve the simultaneous equation then .
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