Q. 16

Prove th

Answer :

Let a5b7 occurs in the (r + 1)th term, in the expansion of (a - 2b)12 given by I2Cr. a12 – r(-2b)r. Then 12 – r = 5. This gives r= 7. 
Thus the coefficient of a5b7 is

12C5 (-2)7 = (-128) = (792) (-128) = -101376

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