The Question
$\angle$ between $\vec{a}$ and $\vec{b}$ is $60^{\circ}$. If $|a|=3$ and $|b|=5$, find $|a+b|$ and $|a-b|$. Hint use law of cosines
Before I used law of cosines, I did something like below, but didn't get the right asnwer:
Is the method wrong or something?