Let f(x)=sin2(x+α)+sin2(x+β)−2cos(α−β)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(α−β)cos(x+α)sin(x+β)−2cos(α−β)sin(x+α)cos(x+β) =sin(2x+2α)+sin(2x+2β)−2cos(α−β)sin(2x+α+β) =2sin(2x+α+β)cos(α−β)−2cos(α−β)sin(2x+α+β) =0 ∴f(x) is a constant function.