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Gopal Menon

· started a discussion

· 1 Months ago

plz explain in detail (Question: 273513.0 )

plz explain in detail

Question:

Find the value of cos10°. cos30°. cos50°. cos70° = ?

Options:
A)

316    

B)

316    

C)

38    

D)

38    

Solution:

Ans: (b)

  

Knowledge Expert

· commented

· 1 Months ago

Dear Student,
Solve as below,
Cos 10 cos 30 cos 50 cos 70 = cos 30 ( cos 10 cos 50) cos 70
root 3 / 2 * 1/2(2cos 10 cos 50) cos 70
root 3/4* (cos60 +cos 40) cos70 [ 2cosx cosy = cos(x+y) + cos(x-y)]
root 3/4 * ( cos60 cos 70 + cos40 cos 70)
use same identity again
.
.
.
you will get = root 3/8 * root 3/2 = 3/16

Thanks and Regards
Team TR

MUKUL SAROHA

· commented

· 1 Months ago

Thnx Abhyut you are adbhut..!!

Abhyut

· commented

· 1 Months ago

Cosx. Cos (60-x). Cos(60+x) = 1/4 Cos3x.
This is an identity. You can google for proof.
put x=10, you will get the answe
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