二項式定理　中四 A.maths

通項 = 8Cr*(x^2)^(8 - r)(-3 / x)^r
= 8Cr*x^(16 - 2r)(-3)^r * x^(-r)
= 8Cr* (-3)^r * x^(16 - 2r - r)
設16 - 2r - r = 7. 則
16 - 3r = 7
3r = 9
r = 3
x^7 的係數 = 8C3* (-3)^3
= 56 * (-27)
= -1512.