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$Sum of the last 20 coefficients in the expansion of (1 + x)^{39}, when expanded$ $in ascending powers of x, is$

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219

218

40C20−219

238

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detailed solution

Correct option is D

There are 40 terms in the expansion of (1+x)39 :Sum of last 20 coefficients is S=39C20+39C21+…+39C38+39C39⇒S=39C19+39C18+…+39C1+39C0 Adding the above two expansions we get 2S=39C0+39C1+…+39C39=239⇒S=238.

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Sum of the last 20 coefficients in the expansion of (1+x)39 , when expanded in ascending powers of x, is

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$Sum of the last 20 coefficients in the expansion of (1 + x)^{39}, when expanded$ $in ascending powers of x, is$