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sin25^@cos115^@=1/2(sin40^@-1)...

`sin25^@cos115^@=1/2(sin40^@-1)`

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`L.H.S =sin 25^{circ} cos 115^{circ}`

`=sin 25^{circ} cos (180^{circ}-65^{circ}) `

`=-sin 25^{circ} cos 65^{circ} `

`=frac{-1}{2}(2 sin 25^{circ} cos 65^{circ})`

`=frac{-1}{2}(sin (25^{circ}+65^{circ})+sin (25^{circ}-65^{circ})) `

`=frac{-1}{2}(sin 90^{circ}+sin (-40^{circ})) `

`=frac{-1}{2}(1-sin 40^{circ}) `

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