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Question

If sinx + cosx = 15 then the value of cos2x



Solution

The correct option is B


sinx + cosx = 15

Squaring both sides,

(sinx+cosx)2 = 125

sin2x+cos2x+2sinxcosx=125

1 + sin2x = 125

sin2x = 125 - 1 = -2425

We know, cos22x = 1 - sin22x

= 1 - (2425)2

 = 1 - 576675

 = 49625

cos2x = ± 725

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