Chapter 5 - Trigonometric Identities - Section 5.2 Verifying Trigonometric Identities - 5.2 Exercises - Page 208: 5

$\sin^2\theta+\cos^2\theta=1$

Reference to the figure in the right angled triangle $AB$=Hypotenuse $BC$=Perpendicular $AC$=Base By Pythagorean theorem $BC^2+AC^2=AB^2$ or $\frac{BC^2}{AB^2}+\frac{AC^2}{AB^2}=1$ $\sin^2\theta+\cos^2\theta=1$