+977 For some real number a and some positive integer n, the first few terms in the expansion of (1+ax)n are
1−6x+12x2+cx3+⋯.
Find c.
+410 {(n1)∗a=−6(n2)∗a2=12
{na=−6n(n−1)2∗a∗a=12
{na=−6na∗a(n−1)=24
−6∗a(n−1)=24
a(n−1)=−4
an−a=−4
{a=−2n=3
(1−2x)3=1−6x+12x2−8x3
c=−8.
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