The sum of the coefficients in the binomial expansion  (a−x)5

(a−x)n=C0   nanx0−C1   nan−1x1+C2   nan−2x2−........+(−1)nCn   na0xn ∴(a−x)5=C0   5a5x0−C1   5a4x1+C2   5a3x2−C3   5a2x3+C4   5a1x4−C5   5a0x5                  =a5−5a4x+10a3x2−10a2x3+5ax4−x5 According to problem sum of coefficients are 1−5+10−10+5−1=0