# Camille's question at Yahoo! Answers regarding finding the parameter of a function

#### MarkFL

Staff member
Here is the question:

Calculus tangent line?

If the line 3x-4y=0 is tangent in the first quadrant to the curve y=x^3+k then k is
(a)1/2
(b)1/4
(c)0
(d)-1/8
(e)-1/2
Could you please show work because I am so lost
I have posted a link there to this topic so the OP may see my work.

#### MarkFL

Staff member
Hello Camille,

Let's begin by equating the derivative of the curve to the slope of the tangent line. The tangent line may be written as $$\displaystyle y=\frac{3}{4}x$$ hence:

$$\displaystyle \frac{3}{4}=3x^2$$

$$\displaystyle x^2=\frac{1}{4}$$

Since we are interesting in the first quadrant solution, we take the positive root:

$$\displaystyle x=\frac{1}{2}$$

Now, the tangent line and the curve have then tangent point in common, and so using the tangle line, we know this point is:

$$\displaystyle \left(\frac{1}{2},\frac{3}{4}\cdot\frac{1}{2} \right)=\left(\frac{1}{2},\frac{3}{8} \right)$$

Hence, we must have:

$$\displaystyle y\left(\frac{1}{2} \right)=\left(\frac{1}{2} \right)^3+k=\frac{1}{8}+k=\frac{3}{8}$$

And so we must have:

$$\displaystyle k=\frac{1}{4}$$

Here is a plot of the curve $$\displaystyle y=x^3+\frac{1}{4}$$ and the tangent line $$\displaystyle y=\frac{3}{4}x$$ for $0\le x\le1$: 