Question

Find the measure of angle between the lines $x+y+7=0$ and $x-y+1=0$.

Answer 1

$x+y+7=0$

$m_1=\frac{-1}{1}$

$x-y+1=0$

$m_2=\frac{-1}{-1}=1$

Product of these two slopes is $-1$, therefore, the lines are at right angles.