If In=∫sinnxcosxdx, then In+In−2 is:
(where n∈N,n≥2 and 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−1cos(n+1)x+c
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D
2n−1cos(n−1)x+c
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Solution
The correct option is A−2n−1cos(n−1)x+c Given: In=∫sinnxcosxdx
Let us consider In+In−2 ⇒In+In−2=∫sinnx+sin(n−2)xcosxdx ⇒In+In−2=∫2sin(n−1)x⋅cosxcosxdx ⇒In+In−2=∫2sin(n−1)xdx ⇒In+In−2=−2n−1cos(n−1)x+c