The coefficient of x 7 in the expression 1 + x 10 + x 1 + x 9 + x 2 1 + x 8 + . . . . + x 10 , is

Question

Aicharya · Accepted Answer

As the given series is in G.P., so its sum is 1 + x 10 1 - x 1 + x 11 1 - x 1 + x 1 + x 10 1 + x 11 - x 11 1 + x 11 &#215; 1 1 + x = 1 + x 11 - x 11 Coefficient of x 7 , is 11 C 7 = 11 C 4 = 330

The coefficient of x 7 in the expression 1 + x 10 + x 1 + x 9 + x 2 1 + x 8 + . . . . + x 10 , is

Solution