If $$a = \widehat i - \widehat j + \widehat k$$, $$b = 2\widehat i + 3\widehat j + 2\widehat k$$ and $$c = \widehat i + m\widehat j + n\widehat k$$ are three coplanar vectors and $$\left| c \right| = \sqrt 6 $$, then which one of the following is correct?

Let ABCD be a parallelogram whose diagonals intersect at P and let O be the origin. What is OA + OB + OC + OD equal to ?

ABCD is a quadrilateral whose diagonals are AC and BD. Which one of the following is correct?

If a $$\times$$ b = c and b $$\times$$ c = a, then which one of the following is correct