JEE Main20207 Jan 2020Evening ShiftMathematicsBinomial TheoremActual

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

Options

A210
B330
C120
D420

Correct answer

B. 330

Step-by-step solution

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 × 1 1 + x = 1 + x 11 - x 11 Coefficient of x 7 , is 11 C 7 = 11 C 4 = 330

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