If the magnitude of sum of two vectors is equal to the magnitude of...

$\left|\overset{\to}{A} + \overset{\to}{B}\right| = \left|\overset{\to}{A} - \overset{\to}{B}\right|</p><p>$\overset{\to}{A}2 + \overset{\to}{B}2 + 2 \left|\overset{\to}{A}\right|\left|\overset{\to}{B}\right|cos\theta = \overset{\to}{A}2 + \overset{\to}{B}2 - 2\left|\overset{\to}{A}\right|\left|\overset{\to}{B}\right|$cos\theta$

$4\left|\overset{\to}{A}\right|\left|\overset{\to}{B}\right|cos\theta =0$

$cos\theta = 0$

$\theta = 90 °$