Chapter 14 Trigonometric Graphs, Identities, and Equations - Investigating Algebra Activity - 14.3 Investigating Trigonometric Identities - Draw Conclusions - Page 923: 10

$\tan(x)=\frac{\sin(x)}{\cos(x)},\cos(-3x)=\cos(3x),\sin(-x)=-\sin(x),\cos(x-\pi/2)=\sin(x)$.

Using the definition of a trigonometric identity, the following are identities among the studied equations: $\tan(x)=\frac{\sin(x)}{\cos(x)},\cos(-3x)=\cos(3x),\sin(-x)=-\sin(x),\cos(x-\pi/2)=\sin(x)$.