(sin A – cos A) ² – 1 = ….

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(sin A – cos A) ² – 1 = ….

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(sin A – cos A)²- 1 = ..
= sin² A – 2 sin A cos A + cos² A – 1
= (sin² A + cos² A) – 2 sin A cos A – 1
= 1 – 2sin A cos A -1
= – 2 sin A cos A
= – sin 2A