graph the expression 1-tan^2x/1+tan^2x, and make a conjecture about another expression that is equivalent to another.
Let us graph the function y=1−tan2x1+tan2x:y=\frac{1-\tan^2x}{1+\tan^2x}:y=1+tan2x1−tan2x:
It follows from the graph that y=cos(2x).y=\cos(2x).y=cos(2x).
Indeed, 1−tan2x1+tan2x=1−tan2x1cos2x=cos2x(1−sin2xcos2x)=cos2x−sin2x=cos(2x).\frac{1-\tan^2x}{1+\tan^2x} =\frac{1-\tan^2x}{\frac{1}{\cos^2 x}} =\cos^2x(1-\frac{\sin^2x}{\cos^2x}) =\cos^2x-\sin^2 x=\cos(2x).1+tan2x1−tan2x=cos2x11−tan2x=cos2x(1−cos2xsin2x)=cos2x−sin2x=cos(2x).
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