If In=∫cosnxcosxdx where n>1 and n∈N, then In+In−2=
(where c is constant of integration)
A
−2n−1cos(n−1)x+c
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B
2n−1cos(n−1)x+c
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C
2n−1sin(n−1)x+c
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D
−2n−1sin(n−1)x+c
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Solution
The correct option is C2n−1sin(n−1)x+c Given: In=∫cosnxcosxdx ⇒In+In−2=∫cosnx+cos(n−2)xcosxdx⇒In+In−2=∫2cos(n−1)xcosxcosxdx⇒In+In−2=∫2cos(n−1)xdx⇒In+In−2=2n−1sin(n−1)x+c