Checkout JEE MAINS 2022 Question Paper Analysis : Checkout JEE MAINS 2022 Question Paper Analysis :

If in the expansion of (1 + x)21, the coefficients of xr and xr+1 are equal, then r is equal to

1) 9

2) 10

3) 11

4) 12

Solution: (2) 10

Given the coefficients of xr and xr+1are equal, 21Cr = 21Cr+1

If nC5 = nCk then s + k = n or s = k

r + r + 1= 21

⇒ 2r = 20

r = 10

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