Algebra: A Combined Approach (4th Edition)

Slope between $(1,1)$ and $(-3,0)$ $\frac{0-1}{-3-1}=\frac{-1}{-4}=\frac{1}{4}$ Slope between $(-4,4)$ and $(-3,0)$ $\frac{0-4}{-3-(-4)}=\frac{-4}{1}=-4$ A right angled triangle has a right angle, meaning two sides are perpendicular to each other. One slope must be the negative reciprocal of the other. $-4$ is the negative reciprocal of $\frac{1}{4}$. Therefore, this forms a right angled triangle.