The coefficient of x5 in the expansion of x2−x−25 is

(x−2)5(x+1)5 = 5C0x5−5C1x4×2+5C2x3×22-5C3x2×23+5C4x×24-5C5×25 5C0+5C1x+5C2x2+5C3x3+5C4x4+5C5x5⇒Coefficient of x5    =5C05C5−5C1×2×5C4+5C2×22×5C3−5C3×23×5C2+5C4×24×5C1−5C5×25×5C0 =1−5×5×2+10×10×4−10×10×8+5×5×16−32 = -81