$2$: $5$ vectors are forming a loop as following figure. All vector's magnitudes are constant. To calculate $V_r,$ prepare the following equations. $($Total $10$ pt $)$

$2\mathrm{.}1$: Loop equations: Prepare two equations fo find $V_r.(5$ pt $)$

$2\mathrm{.}2$: Prepare equations to find $V_r$ by differentiate the loop equations. $(5$ pt$)$