If (1+x2)(1+x2+x4)(1+x2+x4+x6)....(1+x2+x4+....x2n)=n(n+1)∑k=0akxk, then a0+a2+a4+....+an(n+1) equals to
A
12(n+1)!
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B
12(2n)!
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C
(n+1)!
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D
(n!)22
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Solution
The correct option is C(n+1)! (1+x2)(1+x2+x4)........(1+x2+.........+x2n)=n(n+1)∑k=0ak.xk 1.2.3.4.............(n+1)=a0+a1+a2+........+an(n+1) ...(i) 1.2.3.4.............(n+1)=a0–a1+a2+........+an(n+1) ...(ii)
Add (i) and (ii) 2{a0+a2+a4+........+an(n+1)}=2(n+1)! a0+a2+a4+........+an(n+1)=(n+1)!