If (sin−1x)2−(cos−1x)2=a;0<x<1,a≠0, then the value of 2x2−1 is:
A
cos(2aπ)
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B
sin(4aπ)
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C
cos(4aπ)
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D
sin(2aπ)
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Solution
The correct option is Dsin(2aπ) (sin−1x)2−(cos−1x)2=a ⇒(sin−1x+cos−1x)(sin−1x−cos−1x)=a ⇒π2(π2−2cos−1x)=a ⇒π2−2cos−1x=2aπ
Taking sine on both sides, we have sin(π2−2cos−1x)=sin(2aπ) ⇒cos(2cos−1x)=sin(2aπ) ⇒2cos2(cos−1x)−1=sin(2aπ) ∴2x2−1=sin(2aπ)