✔ 最佳答案
(2x - x^2)^12
= 12C0 * (2x)^12 + 12C1 * (2x)^11 * (-x^2) + 12C2 * (2x)^10 * (-x^2)^2 + ... +
12C11 * (2x) * (-x^2)^11 + 12C12 * (-x^2)^12
要找x^15的系數, 設r 為Binomial Expansion 的第 (r + 1) 項, 0 <= r <= 12,
然後考慮算式 (12 - r) + 2r = 15, r + 12 = 15, r = 3
12C3 * (2x)^9 * (-x^2)^3
= 12! / (3! 9!) * (-2^9) * x^15
= 220 * (-512) * x^15
= -112640 * x^15
x^15的系數是-112640.