Wednesday, April 3, 2019

trigonometry - Verifying the trigonometric identity $cos{x} - frac{cos{x}}{1 - tan{x}} = frac{sin{x} cos{x}}{sin{x} - cos{x}}$



I have the following trigonometric identity




$$\cos{x} - \frac{\cos{x}}{1 - \tan{x}} = \frac{\sin{x} \cos{x}}{\sin{x} - \cos{x}}$$



I've been trying to verify it for almost 20 minutes but coming up with nothing



Thank you


Answer



Observe that we need to eliminate $\tan x$



So, using $\tan x=\frac{\sin x}{\cos x},$




$$\cos x-\frac{\cos x}{1-\tan x}$$



$$=\cos x\left(1-\frac{1}{1-\frac{\sin x}{\cos x}}\right)$$



$$=\cos x\left(1-\frac{\cos x}{\cos x-\sin x}\right) (\text{ multiplying numerator & denominator by }\cos x)$$



$$=\cos x\left(1+\frac{\cos x}{\sin x-\cos x}\right)$$



$$=\cos x\left(\frac{\sin x}{\sin x-\cos x}\right)$$



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