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OJASWI KOTHARI

· started a discussion

· 1 Months ago

how?? (Question: 277489.0 )

From where did 1/4sin3theta come? Is it a formula or what?

Question:

The value of 2 + sin 10º sin 50º sin 70º = ?

Options:
A)

78    

B)

18    

C)

98    

D)

178    

Solution:

Ans: (d)

  

Knowledge Expert

· commented

· 1 Months ago

Dear Student,

We can write SinAsin (60-A)sin(60+A)=1/4sin3A
L.H.S. = sin A sin (60° - A) sin (60° + A)
= sin A [ sin² 60° - sin² A] {sin (A + B) sin (A - B) = sin² A - sin² B }
= sin A [3/4 - sin² A]
= (3sin A)/4 - sin³ A = 1/4 [3sin A - 4sin³ A]
= 1/4sin³ A = R.H.S.

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