To evaluate for tan 5x/4 we proceed as follows
5x/4
=5/4*150
=225
which is in quadrant III.
This implies that the value of tan will be positive, hence:
tan 225=tan (270-225)
tan 225=tan 45
using right angle with length of the legs being unit, then
tan 45=1
hence
tan 5x/4=1