oh they're both technically right I think it just depends on how you look at it that's all
If you think of a cos graph between 0 and 2pi, cosx = 0 when x = pi/2, 3pi/2. Now you can either do x = (pi/2 + 2kpi), (3pi/2 +2kpi), or you can just simplify it as x = pi/2 + kpi as 3pi/2 is just pi/2 +...