The coefficient of xr[0≤r≤n−1] in the expression of (x+2)n−1+(x+2)n−2.(x+1)+(x+2)n−3.(x+1)2+...+(x+1)n−1 is
A
nCr(2r−1)
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B
nCr(2n−r−1)
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C
nCr(2r+1)
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D
nCr(2n−r+1)
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Solution
The correct option is AnCr(2n−r−1) Given expression is equal to ∑np=1(x+3)(n−p)×(x+2)(p−1) Which is equal to (x+3)(n−1)×(∑np=1((x+2)(x+3))(p−1))=(x+3)n−(x+2)n So,coefficient of xrisnCr×(3(n−r)−2(n−r))