# Thread: Find The Tangent Line Equation

1. ## Find The Tangent Line Equation

I want to be sure I did this right
I had to find the equation of the tangent line I was given

$f(x)=10-cos(3x)$ at $x=\pi$

So I take the derivative with respect to x with meaning all I had to really do is take the derivative of the function so that gives me

$\frac{dy}{dx}=3sin(3x)$

which this gives me my slope when i plug in x
which gives me 0

and to find the y of the coords. I plug in x in the originial function and get 11

and when i do point slope form i get

y = 11 which cant be correct or is it?

2. Originally Posted by sk8erboyla2004
I want to be sure I did this right
I had to find the equation of the tangent line I was given

$f(x)=10-cos(3x)$ at $x=\pi$

So I take the derivative with respect to x with meaning all I had to really do is take the derivative of the function so that gives me

$\frac{dy}{dx}=3sin(3x)$

which this gives me my slope when i plug in x
which gives me 0

and to find the y of the coords. I plug in x in the originial function and get 11

and when i do point slope form i get

y = 11 which cant be correct or is it?
I followed the same method as you and got the same answer. Plotting the graph does rest on the line y=11

3. Thanks mate, appreciated I dont want to continue on with something If I am doing it wrong