Angle between two lines using two gradients where one is infinite

Question

I am trying to find the angle between two lines; I found the gradient of the first line which was 0.9. I tried to find the gradient of the second line but i got n/0, which is obviously infinite.

I was wondering, what I should do in this situation?

Many thanks,

Mighty Garlic

Answer 1

Hint: $\pm\infty=\lim_{\varepsilon\to 0^+}\tan\pm\left(\frac{\pi}{2}-\varepsilon\right)$.