## Algebra and Trigonometry 10th Edition

The trigonometric Identity is verified. $cos^2β-sin^2β=2~cos^2β-1$
We know that: $cos^2β+sin^2β=1$ $cos^2β-1=-sin^2β$ Start al the left side of the equation: $cos^2β-sin^2β=cos^2β+cos^2β-1=2~cos^2β-1$