# Horizontal Tangent

1. May 31, 2014

### mill

If the tangent is horizontal, it is where the tangent is zero. In single var. calc. that would be at max. or min. for example. I am confused about what horizontal tangent refers to when I am given a parametric equation.

E.g. At what value of t does x=t^2 -t and y=t^2 +t have a horizontal tangent?

The answer is -1/2 which can be found by setting y'=0. I don't understand why this happens though. As in, why dy/dt rather than dy/dx or why does dx/dt not apply? In describing the curve, what is the relationship between the two (x and y given in parametric form) that I could just only look at dy/dt?

My first instinct was to look for dy/dx which would look something like (2t+1)/(2t-1).

Last edited: May 31, 2014
2. May 31, 2014

$\frac{dy}{dx}$ =$\frac{\frac{dy}{dt}}{\frac{dx}{dt}}$.

So for $\frac{dy}{dx}$ to be zero, the numerator i.e $\frac{dy}{dt}$ must be zero. And hence the answer.

3. May 31, 2014

### mill

Thanks.

4. May 31, 2014

### SteamKing

Staff Emeritus
More accurately, if the tangent is horizontal, it is where the slope of the tangent line is zero.