+0  
 
0
118
1
avatar

What is the smallest distance between the origin and a point on the graph of y = 1/2*(x^2 - 8).

 Feb 20, 2023
 #1
avatar+118623 
+1

I want to find the smallest distance between  \((x,\frac{x^2-8}{2}) \qquad and \qquad (0,0)\)

 

The distance between these is

 \(d=\sqrt{(x-0)^2+(\frac{x^2-8}{2}-0)^2} \\d= \sqrt{x^2+(\frac{x^2-8}{2})^2}\)

 

We need to minimize that expression and solve for x

I will get the same result if I minimize 

 \(x^2+(\frac{x^2-8}{2})^2 \qquad or \qquad 4x^2+(x^2-8)^2\)

 

Let

\(T=4x^2+(x^2-8)^2\\ \frac{dT}{dx}=8x+2(x^2-8)^1*2x\\ \frac{dT}{dx}=8x+4x(x^2-8)\\ \frac{dT}{dx}=8x+4x^3-32x\\ \frac{dT}{dx}=4x^3-24x\\ \frac{dT}{dx}=4x(x^2-6)\\ \frac{dT}{dx}=4x(x-\sqrt 6)(x+\sqrt 6)\\~\\ \frac{dT}{dx}=0 \;\;\;\;when\;\;\;\;\ x=0,\;\;x=\pm\sqrt6\)

 

When x=0 

     \(d= \sqrt{x^2+(\frac{x^2-8}{2})^2}\\ d= \sqrt{16}\\ d=4 \)

 

When   \(x=\pm\sqrt6\)

 

\(d= \sqrt{x^2+(\frac{x^2-8}{2})^2}\\ d= \sqrt{6+(\frac{6-8}{2})^2}\\ d= \sqrt{6+1}\\ d=\sqrt7\\ d\approx 2.64\)

 

So it looks to me like the min distance is  when x= \(\sqrt7 \quad \text{and it occurs when }\quad x=\pm\sqrt6\)

 

I'll look at the second derivative for verification.

\(\frac{dT}{dx}=4x^3-24x\\ \frac{d^2T}{dx^2}=12x^2-24\\ \frac{d^2T}{dx^2}=12(x^2-2)\\ \)

 

 

When    \(x=\pm \sqrt 6\\ \frac{d^xT}{dx^2}= 48>0 \quad \text{So minimum}\)

 

 

When    \(x= 0\\ \frac{d^xT}{dx^2}= -24<0 \quad \text{So maximum}\)

 

So I get the minimum distance to be    sqrt7

 

 

And here is the pic

 

 

 

 

 

 

 

LaTex:

T=4x^2+(x^2-8)^2\\
\frac{dT}{dx}=8x+2(x^2-8)^1*2x\\
\frac{dT}{dx}=8x+4x(x^2-8)\\
\frac{dT}{dx}=8x+4x^3-32x\\
\frac{dT}{dx}=4x^3-24x\\
\frac{dT}{dx}=4x(x^2-6)\\
\frac{dT}{dx}=4x(x-\sqrt 6)(x+\sqrt 6)\\~\\
\frac{dT}{dx}=0 \;\;\;\;when\;\;\;\;\ x=0,\;\;x=\pm\sqrt6

 Feb 21, 2023

4 Online Users

avatar
avatar