If you have a vertical line like that, what is your rate of change of y with respect to x? Well, some people might say it's infinity, or you could say it's undefined. It's undefined in some way, because at some point, or one way to think about it, you're gonna try to divide by zero because you have a huge change in y over no change in x. Or another way to think about it that's a little bit more in line with that is you could say that your change in x with respect to change in y, notice I took the reciprocal, so now we're talking about a change in the derivative of x with respect to y is equal to zero. Because your y can change, but as your y changes, your x does not change. So can we use this little insight here on vertical lines to think about the coordinates of all points on the curve at which the line tangent to the curve at that point is vertical? Well, before, they told us, they gave us the curve of the equation, and they also told us what dy dx is.