I haven't done trig in awhile, but I think you got it wrong.
5) t = 8pi
Since 8pi is just 4*2pi, we are at the point (1,0) on the unit circle.
6) t = -7pi/2
Convert that to degrees to get -630 degrees which is the same thing as 90 degrees. 90 degrees on the unit circle is the point (0,1).
