Find the coefficient of x11 in the expansion (1−6x6)(1−x)−7
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Coefficient of x11 in (x−x6)6(1−x)−7
Ignoring the higher order terms
Coefficient of x11 in (1−6x6)(1−x)−7
Coefficient of x11 in (1−6x6)(1+7C1x+8C2x2+9C3x3+..............) = 17C11−611C6=17C6−611C5