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Mikey's question at Yahoo! Answers regarding finding the tangent point of two functions

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MarkFL

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Feb 24, 2012
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Here is the question:

Calculus Help Please?


Find the point where the graphs of f(x)=x^3-2x and g(x)=0.5x^2-1.5
are tangent to each other; or have a common tangent line.

I'm pretty sure I have to find the derivatives of each and set them equal to eachother. Do I then just solve for x? My answer is seems weird. Can you explain how you would go about this problem please?

Thanks.
I have posted a link there to this topic so the OP can see my work.
 
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MarkFL

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Feb 24, 2012
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Hello Mikey,

If the two given functions are tangent to each other, then the difference between the two functions will have a repeated root:

\(\displaystyle f(x)-g(x)=(x-a)^2(x-b)\)

\(\displaystyle x^3-\frac{1}{2}x^2-2x+\frac{3}{2}=x^3-(2a+b)x^2+\left(a^2+2ab \right)x-a^2b\)

Equating corresponding coefficients, we find:

\(\displaystyle 2a+b=\frac{1}{2}\)

\(\displaystyle a^2+2ab=-2\)

\(\displaystyle a^2b=-\frac{3}{2}\)

The third equation implies:

\(\displaystyle b=-\frac{3}{2a^2}\)

Using this, the first equation gives:

\(\displaystyle 4a^3-a^2-3=0\implies a=1,\,b=-\frac{3}{2}\)

And so the two functions are tangent to one another at $x=1$. Here is a plot:

mikey.jpg