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Prove that:`sin(pi/10) sin((13pi)/10)=-1/4` |
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Answer» Here, we will use, `sin 18^@ = (sqrt5-1)/4 and cos 36^@ = (sqrt5+1)/4` Now, `L.H.S. = sin(pi/10)sin((13pi)/10)` `=sin18^@sin(pi+(3pi)/10)` `=sin18^@(-sin((3pi)/10))` `=-sin18^@sin54^@` `=-sin18^@sin(90^@-36^@)` `=-sin18^@cos36^@` `=-((sqrt5-1)/4)( (sqrt5+1)/4) ` `=-((5-1)/16)` `=-4/16` `=-1/4 = R.H.S.` |
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