Multiply and divide by , we get

But 2sin A cos A = sin 2A

Then the above equation becomes,

Multiply and divide by 2, we get

But 2sin A cos A = sin 2A

Then the above equation becomes,

Multiply and divide by 2, we get

But 2sin A cos A = sin 2A

Then the above equation becomes,

Multiply and divide by 2, we get

But 2sin A cos B = sin (A+B) +sin(A–B), so the above equation becomes,

Multiply and divide by , we get

But 2sin A cos A = sin 2A

Then the above equation becomes,

Multiply and divide by 2, we get

But 2sin A cos A = sin 2A

Then the above equation becomes,

Hence proved