Recall your sum/difference formulas. You have
sin(2x-y)+sin(y) = 2sin(x)cos(x+y)
cos(2x-y)+cos(y) = 2cos(x)cos(x+y)
Now just divide and you have tan(x)
Prove the identity:
(sin(2x-y)+siny)/(cos(2x-y)+cosy )=tanx
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