Question 1104466
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Prove the identity:

cos^2(y) - sin^2(y) = 1 - 2sin^2(y).
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<pre>
  cos^2(y) - sin^2(y) =      (substitute here cos^2(y) = 1 - sin^2(y) to get) 


= 1 - 2*sin^2(y).
</pre>

QED.