It is known that (r+1)th term (Tr+1) in the binomial expansion of (a+b)n is given by Tr+1=nCrbn−rbr Thus the 4th term in the expansion of (x+2y)12 is T4=T3+1=12C3(x)12−3(−2y)3=(−1)312!3!9!.x9.(2)3.y3 =−12.11.103.2.(2)3x9y3=−1760x9y3
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