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sin (x+y)=cos ((pi)/(2)-(x+y))=cos (((pi...

`sin (x+y)=cos ((pi)/(2)-(x+y))=cos (((pi)/(2)-x)-y))'=cos ((pi)/(2)-x) cos y+sin ((pi)/(2)-x) sin y' =sin x cos y+cos x sin y`

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Replacing .(y). by .-(y)., in the Identity 7 .sin (x+(-y))=sin x cos (-y)+cos x sin (-y).
.sin (x-y)=sin x cos y-cos x sin y.
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