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Question

If sinθ=0.8 and θ lies in 2ndquadrant. Find the value of cosθ.
Given, sin2θ+cos2θ=1

A
0.6
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B
0.6
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C
0.8
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D
0.8
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Solution

The correct option is B 0.6
We have, sinθ=0.8
And we have,
sin2θ+cos2θ=1cos2θ=1sin2θcos2θ=10.82=0.36cos2θ=(0.6)2cos2θ(0.6)2=0(cosθ0.6)(cosθ+0.6)=0cosθ=0.6,0.6

Since θ lies in 2nd quadrant and we know cosθ is negative in 2nd quadrant.
cosθ=0.6

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