## Calculus 8th Edition

$74.7 ^ \circ$
$x+2y=7$ $\implies$ $a=y=-\frac{1}{2}x+\frac{7}{2}$ and $5x-y=2$ $\implies$ $b=y=5x-2$ Thus, $a= \lt 2,-1 \gt$ and $b = \lt 1,5 \gt$ $\theta = cos^{-1}\dfrac{a \cdot b}{|a||b|}=cos^{-1}\dfrac{2(1)+(-1)5}{\sqrt {2^2+(-1)^2}+\sqrt {1^2+5^2}}$ $\approx 105.3 ^ \circ$ Subtract from $180^ \circ$ to get acute angle. $180^ \circ-105.3 ^ \circ=74.7 ^ \circ$