Prove : 4 sin /10 sin 131 /10 = -1....
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Prove : 4 sin /10 sin 131 /10 = -1.

JEE/Engineering Exams
Maths
Solution
128
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( 4 sin frac{pi}{10} cdot sin frac{13 pi}{10} ) ( =4 sin frac{pi}{10} cdot sin left(pi+frac{3 pi}{10}right)-sin ) ( =-4 sin frac{pi}{10} cdot sin frac{3 pi}{10} ) ( =-4 sin frac{pi}{10} int cos sin left(frac{pi}{2}-frac{pi pi}{5}right) ) ( frac{left.-4 sin (pi) cos frac{pi}{5}right) times cos pi}{cos (pi / 10)} ) ( =-2 sin left(frac{pi}{5}right) cdot cos left(frac{pi}{5}right) ) ( cos (pi / 10) ) ( =-sin left(frac{2 pi}{5}right) ) ( cos left(frac{pi}{10}right)>frac{pi}{2}-frac{2 pi}{5} )
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