Slope of a vertical line is?

It is such an interesting question. Slope is obviously infinity, but we cannot define the slope is +ve infinity or -ve infinity. Therefore we always say that it is "undefined".

i'm not very sure...
but we always say that it is "undefined"
because we calculate slope by the change in the y coordinates over the change in the x coordinates between two points on a straight line
i.e. change in y / change in x
for a vertical line, the change in x = 0
it is the same problem about dividing something by 0
you may think in this way, although the smaller the denominator, the larger the slope
pls consider how you divide a cake into pieces
u can divide it into 2 portions, 3, 4 and so on
but it is meaningless to divide it by 0

dunno if i can help u :P

It is very nice that you would read some mathematics book.

Simply answer your question in two ways.

1. Infinity is not a number.

2. The meaning of slope is the vertical increment when there is a horizontal increment. For a vertical line, there is NO horizontal increment at all.