Trigonometry - Basics of Trigonometric Ratios

For a nonvertical straight line, let \(\theta\) be its inclination measured anticlockwise from the positive \(x\)-direction, with \(0\le\theta<\pi\). Its slope is \(m=\tan\theta\), not \(\theta\). A vertical line has \(\theta=\pi/2\) and undefined slope.

slope, \(m=\frac{y_2-y_1}{x_2-x_1}=\tan{\theta}\)