cos(90°-x)×cos(90°-y)-sin(90°-x)×sin(90°-y)=-cos(x+y)
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Step-by-step explanation:
cos(90°-x)×cos(90°-y)-sin(90°-x)×sin(90°-y
=cos(90-x+90-y)
=cos(180-x-y)
=cos{180-(x+y)}
=-cos(x+y)
that is proved.
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