6.2 Trigonometric Functions: Unit Circle Approach/78

$\theta \rightarrow (5,-12)$

$\theta \rightarrow x=5,\,y=-12,\,r=\,?$

${\begin{aligned}(5)^{2}+(-12)^{2}&=r^{2}\\25+144&=r^{2}\\169&=r^{2}\\{\sqrt {169}}&={\sqrt {r^{2}}}\\13&=r\\\end{aligned}}$