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Question

Let f(x)=sin2(x+α)+sin2(x+β)2cos(αβ)sin(x+α)sin(x+β).Which of the following is TRUE about f(x)?

A
f(x) is strictly increasing in x(α,β)
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B
f(x) is strictly decreasing in x(α,β)
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C
f(x) is strictly increasing in x(α,α+β2) and strictly decreasing in x(α+β2,β)
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D
f(x) is a constant function
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Solution

The correct option is D f(x) is a constant function
Differentiating the given function:
f(x)=2sin(x+α)cos(x+α)+2sin(x+β)cos(x+β)2cos(αβ)cos(x+α)sin(x+β)2cos(αβ)sin(x+α)cos(x+β)
=sin(2x+2α)+sin(2x+2β)2cos(αβ)sin(2x+α+β)
=2sin(2x+α+β)cos(αβ)2cos(αβ)sin(2x+α+β)
=0
f(x) is a constant function.

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