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Find the point(s) on the curve $y^3=x^2$ closest to the point $P=(0,4).$ I understand that there is a way to solve this, using the distance formula, however this turns out to seem rather complicated. I am also aware that there is a calculus method to solving this question, however am unsure as to what that method is, exactly. Any help is appreciated. Thanks :)

Riccardo.Alestra
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user341952
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2 Answers2

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The square of the distance between the point $P$ and another point with coordinates $(x,y)$ is $$d^2=x^2+(y-4)^2.$$

Since the curve is defined as a set of points $(x,y)$ related to each other by the relation $y^3=x^2$, we have the square of the distance from $P$ to the curve equal to: $$d^2=y^3+(y-4)^2$$ Thus, the task is to find the smallest $d$ possible, when $d$ is actually a function of $y$. For this, we employ the derivative of $d(y)$ to find extremums of the function: $$d(y)=\sqrt{y^3+(y-4)^2}$$ $$d'(y)=\frac{3y^2+2(y-4)}{2\sqrt{y^3+(y-4)^2}}$$ Now, to find extremums, solve $d'(y)=0$, or in our case $3y^2+2(y-4)=0$. This quadratic equation has two solutions $y=-2$ and $y=\frac{4}{3}$. But the solution $y=-2$ is not acceptable, since $y^3=x^2$, and that would mean $-8=x^2$. Hence, we are left with only one solution and two pairs (do you know why?) of points:

$(\frac{8}{\sqrt{28}},\frac{4}{3})$ and $(-\frac{8}{\sqrt{28}},\frac{4}{3})$.

P.S. I did not check that obtained solution is actually a minimum. Can you validate that?

Edit: Dear reviewer, I want to edit the typo in "suare" to "square". Stupid StackExchange doesn't let me because it's less then 6 symbols!

Boris Month
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Tomas
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  • Thank you! Using the second derivative test I got f''(x)= 6y+2 and then subbed in the value for y, resulting in an answer of 10, thus confirming that the solution is a minimum. – user341952 May 24 '16 at 13:53
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Hint:

The distance is $ \sqrt{x^2+(y-4)^2}$

replace $x^2$ with $y^3$, the distance become $ \sqrt{y^3+(y-4)^2}$

Let $u(y) = y^3+(y-4)^2$ find the stationary point of u(y):

$u'(y) = 3y^2+2y-8 = 0 $ => $y = -2$ or $4/3$

substitute the roots into distance function and choose the minimal one.

References: correlation between stationary points and extrema

Basic identities of derivatives

Zau
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