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Question: If the 21<sup>st</sup> and 22<sup>nd</sup> terms in the expansion of (1 –x)<sup>44</sup> are equal, ...

If the 21st and 22nd terms in the expansion of (1 –x)44 are equal, then x =

A

7/8

B

–8/7

C

–7/8

D

None

Answer

–7/8

Explanation

Solution

T21 = T22

Ž T20+1 = T21+1

Ž 44C20 x20 = 44C21 (–x)21

Ž x =44C2044C21\frac{-^{44}C_{20}}{44C_{21}}= (44)!(20)!(24)!\frac{(44)!}{(20)!(24)!}× (21)!(23)!(44)!\frac{(21)!(23)!}{(44)!}

= 2124\frac{- 21}{24}= 78\frac{- 7}{8}