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Q.

If sinx+sin2x=1, then cos2x+cos4x=

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a

2

b

0

c

1

d

-1

answer is B.

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Detailed Solution

Method-I

sinx+sin2x=1

sinx=1sin2x

sinx=cos2x

cos2x+cos4x

cos2x+(cos2x)2

=cos2x+(sinx)2

=1

Method-II

sinx+sin2x=1

sinx=1sin2x

sinx=cos2x

Squaring on both sides

sin2x=cos4x

1cos2x=cos4x

cos2x+cos4x=1

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