## Calculus 8th Edition

$cos^{-1}(\frac{5}{ \sqrt {26}\times \sqrt {30}})\approx 79.7^\circ$
The dot product of $a=a_{1}i+a_{2}j+a_{3}k$ and $b=b_{1}i+b_{2}j+b_{3}k$ is defined as $a.b=a_{1}\times b_{1}+a_{2}\times b_{2}+a_{3}\times b_{3}$ The magnitude of a vector is given as $|a|=\sqrt {(a_{1})^{2}+(a_{2})^{2}+(a_{3})^{2}}$ $a.b=-5+6+4=5$ $|a|=\sqrt {(-1)^{2}+(3)^{2}+(4)^{2}}=\sqrt {26}$ $|b|=\sqrt {(5)^{2}+(2)^{2}+(1)^{2}}=\sqrt {30}$ Angle between two vectors is given by $cos\theta=\frac{a.b}{|a||b|}$ $cos\theta=\frac{5}{ \sqrt {26}\times \sqrt {30}}$ $\theta=cos^{-1}(\frac{5}{ \sqrt {26}\times \sqrt {30}})\approx 79.7^\circ$ Hence, $cos^{-1}(\frac{5}{ \sqrt {26}\times \sqrt {30}})\approx 79.7^\circ$