Question

If a and b are two vectors such that



a.B<0 and



a.B=a×b, then the angle between vectors a and b is

Answer 1

$a.b = a \times b \\ |ab| \cos( \alpha ) = |ab| \sin( \alpha ) \\ \cos( \alpha ) = \sin( \alpha ) \\ \alpha = \frac{\pi}{4}$
in first quadrant.