The output of
Math.cos(90 * Math.PI/180)
is
6.123031769111886e-17
Notice the e-17 at the end, which means that this number is 6.123 x 10-17. This is a number so vanishingly close to 0 that it's effectively 0. The reason that it's not identically 0 is due to rounding errors in the IEEE-754 double format which prevents you from getting an exact representation of π / 2 and causes minute rounding errors in the calculation of the cosine.
By the way - I was pretty surprised as well when the result came back starting with a 6! It's only after I looked at the very end that things started to make sense.
Hope this helps!
