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The difference of angles is sharp. The right side has no , so first we want to cancel in left side.
We see we can factor from , and from . Thus we can simplify and . By using the Triple Angle formulas we get:
Being very straight forward, we want to factor out and , at the top of the two fractions respectively, and cancel it with the bottom.
Then combine the like terms, we have
We know can help us to cancel , so we factor out -4, we get,
The identity has been proven.
It's not hard right? Without knowing our idea you can do it too, but can you give another proof?
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