1.

Coefficient of x^5 in ( 1+ x)^21 + (1 + x)^22 + …….+ (1 + x)^30 is

Answer»

`""^51C_5`
`""^9C_5`
`""^31C_6 - ""^21C_6`
`""^30C_5 + ""^20C_5`

ANSWER :C


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