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# What is the shortest distance from the origin to the circle?

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What is the shortest distance from the origin to the circle defined by $$x^2-24x +y^2+10y +160=0$$?

The equation can also be written as $$\left(x-12\right)^{2\ }+\ \left(y+5\right)^2=9$$, so we know that the radius of the circle is 3, and the origin is at (12,-5).

Guest Sep 23, 2018
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I think the easiest way to do this is to parameterize the circle via a single variable and solve the minimization problem.

$$\text{let }p \text{ be a point on the circle given by }(x-12)^2 + (y+5)^2 = 9 \\ \text{then p can be described as }\\ p=(3 \cos(\theta)+12,~3\sin(\theta)-5)$$

$$\text{The squared distance of this point to the origin is }\\ d^2 = (3 \cos(\theta)+12)^2 + (3\sin(\theta)-5)^2 =\\ 9 \sin ^2(\theta )-30 \sin (\theta )+9 \cos ^2(\theta )+72 \cos (\theta )+169 = \\ -30 \sin (\theta )+72 \cos (\theta )+178 = \\ 78 \cos(\theta+\arctan(5/12))+178$$

$$d^2 \text{will clearly be minimized when }\\ \theta + \arctan(5/12) = \pi\\ \theta = \pi - \arctan(5/12)\\ \text{at this angle }d^2 = -78 + 178 = 100\\ d=10$$

Rom  Sep 23, 2018