垂直線的傾角係幾多?

inclination of a vertical line= 90°
Why you think that is underfined....that is because you confuse with tan90°

Since lim(x->90°) tan(x)=infinity.
You can refer to the graph of y=tanx , you can see that tanx become closer and closer to infinity when x become closer and closer to 90°
So we say that tan90° is undefined , but inclination of a vertical line must equal 90°

Inclination is 90 degree. Slope is tan 90 which is undefined.