Trigonometry 7th Edition

$1 - 2\cos\theta\sin\theta$
Given expression- $(\cos\theta-\sin\theta)^{2}$ = $\cos^{2}\theta -2\cos\theta\sin\theta + \sin^{2}\theta$ (Recall $(a+b)^{2} = a^{2} -2ab + b^{2}$) = $\cos^{2}\theta + \sin^{2}\theta - 2\cos\theta\sin\theta$ = $1 - 2\cos\theta\sin\theta$ ( From first Pythagorean identity $\cos^{2}\theta + \sin^{2}\theta$ = 1 )