If Cosv = 0 equals 90°+ k * 180°, wht can't I write: +/-90°+ k * 180°

Why is that ?

Cosv = 0 does not "equal" 90 + k * 180,
what you mean is that Cos(v) = 0 has the solution set
v = 90 degrees + k * 180 degrees.

OK, and you want to know why you can't write
v = plus or minus 90 degrees + k * 180 degrees.
Actually you CAN, but it's redundant.
The solution set
v = 90 deg + k * 180 deg
already includes all the negative values of k as well as the positive ones.
All we assume about k is that it's an integer.