Question

How do you evaluate sec 90?

Answer 1

I think it is in degrees...
So you get that `sec(90°)=1/cos(90°)=1/0` which cannot be calculated.
You can get "near" `90°` from the left or the right using, say, the limit. You'll find that near `90°` the `sec` tends to `oo`.
Graphically (blue line on `pi/2=90°`):