lf (1−x−x2)20=40∑r=0arxr, then a2+2a4+3a6+…………+20a40=
A
20
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B
40
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C
10
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D
-10
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Solution
The correct option is D 10 The value of a2+2a4+3a6...+20a40 It is given by, f′(1)−f′(−1)4 ...(i) Since (1−x−x2)20=∑40r=0arxr f(x)=(1−x−x2)20 Hence f′(x)=−20(1−x−x2)19(2x+1) Therefore f′(−1)=20 f′(1)=60 Substituting in equation (i), we get f′(1)+f′(−1)4 =404 =10