If prove that 1–2 sin²A = 2cos²A – 1.

Let the third side be p,

By Pythagoras theorem,

⇒ 8² + p² = 17²

⇒ 64 + p² = 289

⇒ p² = 225

⇒ p = 15

Therefore, the other angles are:

sin θ =

cos θ =

Putting these values in the left hand side:
⇒ 1–2×

⇒ 1–2×

⇒ 1–

⇒

Putting values in the right hand side:

⇒ 2×

⇒ 2× –1

⇒

⇒

Therefore, Left hand side and right hand side are equal.

Hence proved.