Question

prove the identity: cos((pi/2)-x)= sinx

Answer 1

By cos(α-β)=cosα·cosβ+sinα·sinβ,
cos(pi/2-x)=cos(pi/2)·cos(-x)+sin(pi/2)·sin(-x)
=0·cos(-x)+(-1)·sin(-x)
=0+(-1)·sinx
=-sinx

Answer 2

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