Chapter 2 - Section 2.5 - Equations of Lines and Linear Models - 2.5 Exercises - Page 230: 74

$m_{1}m_{2}=-1$

In Exercise 73, we got: $-2x_{1}x_{2}(m_{1}m_{2}+1)=0$ This means that: $-2x_{1}=0$ or $x_{2}=0$ or $(m_{1}m_{2}+1)=0$ However, we know that $x_1$ and $x_2$ cannot be zero (they are not located at the origin, not at the x-intercepts because they are not horizontal). So we have: $m_{1}m_{2}+1=0$ $m_{1}m_{2}=-1$