I think that you made a mistake in the dot product. I get .
Recall that if the dot product is negative the angle is obtuse.
two forces N and N are given by
a) calculate, correct to the nearest degree, the angle between the directions of and
so using the dot product formula
I end up with
I think i have done this right however the angle i inverse cos is negative, so do I draw up a cast diagram to find the angle? if so is it the angle 180 - 72.98?