Find the coefficient of x 5 in the product (1 + 2 x ) 6 (1 – x ) 7 using binomial theorem.

Using Binomial Theorem, the expressions, (1 + 2x)6 and (1 – x)7, can be expanded as

The complete multiplication of the two brackets is not required to be carried out. Only those terms, which involve x5, are required.

The terms containing x5 are

Thus, the coefficient of x5 in the given product is 171.

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