92. When two vectors A and B are drawn from a common point, the angle between them is $\phi .$ (a) Using vector techniques, show that the magnitude of their vector sum is given by $$ \sqrt{A^{2}+B^{2}+2 A B \cos \phi} $$ (b) If A and B have the same magnitude, for which value of $\phi$ will their vector sum have the same magnitude as A or B?

 92. When two vectors A and B are drawn from a common point, the angle between them is $\phi .$ (a) Using vector techniques, show that the magnitude of their vector sum is given by

$$

\sqrt{A^{2}+B^{2}+2 A B \cos \phi}

$$

(b) If  A and B have the same magnitude, for which value of $\phi$ will their vector sum have the same magnitude as A or B?