`y = x^2 + 9` Determine the point(s) (if any) at which the graph of the function has a horizontal tangent line.

1 Answer

zach9414's profile pic

zach9414 | High School Teacher | (Level 2) Adjunct Educator

Posted on

So I will start by rewriting this equation in the form:

`f(x)=x^2+9 `

We know that if we have `f'(x)` then we can plug in any value for x, and the result will be what the slope of the function is at that given point. 

So if we find the derivative of this original function, we should be left with the following:

`f'(x)=2x `  

If the tangent line to this graph is a horizontal line, then the slope should be 0/x or just 0.

So we can set what we calculated for f'(x) equal to zero and we can solve for our x-value:

`2x=0 `

`x=0 `

So, this tells us that if our "x" value is zero, then the slope should be horizontal. Now that we know the x-value of the coordinate, lets plug this into the original equation to find the y-coordinate!:


Therefore, our coordinates for the point where the tangent line to the graph is horizontal would be (0,9)