Answer
$${\bf{u}} \cdot {\bf{v}} = - 50,\,\,\,\,{\text{and}}\theta = \frac{{3\pi }}{4}$$
Work Step by Step
$$\eqalign{
& {\bf{u}} = \left\langle {10,0} \right\rangle {\text{ and }}{\bf{v}} = \left\langle { - 5,5} \right\rangle \cr
& {\text{find the dot product using the theorem 11}}{\text{.1 }}\left( {page\,\,783} \right) \cr
& {\bf{u}} \cdot {\bf{v}} = \left\langle {10,0} \right\rangle \cdot \left\langle { - 5,5} \right\rangle = \left( {10} \right)\left( { - 5} \right) + \left( 0 \right)\left( 5 \right) \cr
& {\bf{u}} \cdot {\bf{v}} = - 50 \cr
& {\text{find the magnitude of }}{\bf{u}}{\text{ and }}{\bf{v}}\,\,\left( {see\,\,page\,\,\,\,776} \right) \cr
& \left| {\bf{u}} \right| = \left| {\left\langle {10,0} \right\rangle } \right| = \sqrt {{{\left( {10} \right)}^2} + {{\left( 0 \right)}^2}} = 10 \cr
& \left| {\bf{v}} \right| = \left| {\left\langle { - 5,5} \right\rangle } \right| = \sqrt {{{\left( { - 5} \right)}^2} + {{\left( 5 \right)}^2}} = \sqrt {50} = 5\sqrt 2 \cr
& {\text{find the angle between the vectores using }}\cos \theta = \frac{{{\bf{u}} \cdot {\bf{v}}}}{{\left| {\bf{u}} \right|\left| {\bf{v}} \right|}}{\text{ then}} \cr
& \cos \theta = \frac{{{\bf{u}} \cdot {\bf{v}}}}{{\left| {\bf{u}} \right|\left| {\bf{v}} \right|}} = \frac{{ - 50}}{{\left( {10} \right)\left( {5\sqrt 2 } \right)}} \cr
& {\text{simplifying}} \cr
& {\text{cos}}\theta = - \frac{1}{{\sqrt 2 }} \cr
& {\text{solving for }}\theta \cr
& \theta = {\cos ^{ - 1}}\left( { - \frac{1}{2}} \right) \cr
& {\text{simplify by using a calculator}} \cr
& \theta = \frac{{3\pi }}{4} \cr} $$
