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absar hashmi

· started a discussion

· 1 Months ago

sine 18 and cos 36..values.?? how, please somebody explain (Question: 300387.0 )

do we need to memorize?

Question:

The value of sin 36º sin 72º sin 108º sin 144º -

Options:
A)

34    

B)

116    

C)

14    

D)

516    

Solution:

Ans: (d)

  

Knowledge Expert

· commented

· 1 Months ago

Dear Student,
As sin (180° - θ) = sin θ
Therefore sin 108 = sin ( 180 - 72 ) = sin 72
Similarly sin 144 = sin 36

Now again the id that is used in 2 sin^ 2 ( 36 ) is cos(2x) = 1 - 2 sin^2(x)
Therefore 2sin^2 x = 1- cos2x

Now again identity that is used after that is cos( 90 -x ) = sin x.

Hope you understood .

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