Understanding trigonometric identities Can someone help me understand trigonometric identities? For example, it is known that $\cos(90-\theta)$ is equal to $\sin \theta$, and vice versa. But why? Is it something to do with the unit circle? Is it visual?

Take a right triangle $ABC$ with $\angle B=90^\circ$ and $\angle A=\theta$. Then $\sin\theta = \frac{BC}{CA}=\cos \angle C=\cos (90^\circ-\theta)$.
