Two co-terminus vectors $\vec a$ and $\vec b$ include at an angle of 60°. If $|\vec a|=|\vec b|=c$, then the length of the diagonal of the parallelogram formed by $\vec a$ and $\vec b$ that does not include the co-terminus point is

$c$

The length of the required diagonals is $|\vec a -\vec b|$.

Now,

$|\vec a -\vec b|^2=|\vec a|^2+|\vec b|^2-2(\vec a.\vec b)$

$⇒|\vec a -\vec b|^2=|\vec a|^2+|\vec b|^2-2|\vec a||\vec b|\cos 60°$

$⇒|\vec a -\vec b|^2=c^2 + c^2 - c^2 = c^2⇒|\vec a -\vec b|=c$