Answer
$\approx 67.62^{\circ}$ or $1.18$ rad
Work Step by Step
The result of exercise 31 tells us that
${\bf v}=\langle a,\ b \rangle$ is perpendicular to lines $ax+by=c$
${\bf n_{1}} =\langle 12,5\rangle$ is perpendicular to $12x+5y=1$
${\bf n_{2}}= \langle 2,-2\rangle$ is perpendicular to $2x-2y=3.$
$\displaystyle \theta=\cos^{-1}(\frac{{\bf n_{1}}\cdot{\bf n_{2}}}{|{\bf n_{1}}||{\bf n_{2}}|})$
$=\displaystyle \cos^{-1}(\frac{24-10}{\sqrt{144+25}\cdot\sqrt{4+4}})$
$=\displaystyle \cos^{-1}(\frac{14}{13\cdot 2\sqrt{2}})\approx 67.62^{\circ}$ or $1.18$ rad
