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सिद्ध कीजिए कि `sin 25^(@)cos 115^(@)=(1)/(2)(sin 40^(@)-1)`

Answer» माना`" "2sin25^(@)cos115^(@)=2cos115^(@)sin25^(@)`
`=sin(115^(@)+25^(@))-sin(115^(@)-25^(@))`
`=sin(140^(@))-sin90^(@)=sin(180^(@)-40^(@))-1`
`=sin40^(@)-1`


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