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NCERT Class XI Mathematics - Trigonometric Functions - Solutions

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Question : 36
Total: 61
sinx+sin3x
cosx+cos3x
 = tan 2x
Solution:  
We have, L.H.S.=
sinx+sin3x
cosx+cos3x
=
2sin(
x+3x
2
)cos(
x−3x
2
)
2cos(
x+3x
2
)cos(
x−3x
2
)
 =
2sin2xcosx(−x)
2cos2xcos(−x)
 = tan 2x = R.H.S.
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