4楼: Originally posted by Edstrayer at 2015-10-20 03:43:38
\left(x-\frac{2}{x}\right)^n=\sum\limits_{k=0}^nC_n^kx^k\left(-\frac{2}{x}\right)^{n-k}
\sum\limits_{k=0}^nC_n^k=2^n=256\Rightarrow n=8
n-2k=0\Rightarrow k=8/2=4
C_n^kx^k\left(-\frac{2}{x}\right)^ ...