Use the unit circle to find sec (-90)

Using the unit circle, you can visualise the secant as follows:

Since -90 means that the radius points downwards, it means that the radius is vertical as well, and thus it never meets the vertical line through (1,0).

Thus, the secant is not defined at -90.

In fact, if we switch back to the function notation, we can confirm this claim, since we have

which is undefined.

the first one, all those points on the line just make a strait line.

step-by-step explanation:

compare