I’ve drawn a triangle that has an angle close to 90°, it’s looks more like 85°. That is to make it look like a triangle rather than a degenerate triangle (straight line). But I have used the values of the sides for when the triangle has a ninety degree angle.
As you go from 85° to 90°, the adjacent side goes from being small to being zero.
Using SohCahToa, you can get the tangent of theta by dividing the opposite by the adjacent side.
Which is either undefined (in earlier classes) or something like ∞. The limit from the right and left changes the sign though.