Question

Is this equation an identity? Cos²y-Sin²y=Sin(2y)

Answer 1

No. See below.

Explanation:

No. Regardless of how we try to simplify the left side, the identity will not hold true.

The following are identities:

`sin(2y)=2sinycosy`
`cos(2y)=cos^2y-sin^2y`

So, the equation wrongly sets two completely different trigonometric identities equal.