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Geometry Difficulty 3.6 AMC 10/12 Find the answer

In polar coordinates, find the distance from the point P(2,π3)P(2, \frac{\pi}{3}) to the center of the circle given by ρ=2cosθ\rho = 2\cos\theta.

A number or a short expression. Spacing and $ signs are ignored.

Solution

The equation of the circle in polar coordinates is ρ=2cosθ\rho = 2\cos\theta.

1. Convert the polar equation to Cartesian coordinates:
To find the center of the circle in Cartesian coordinates, we start by expressing ρ\rho and θ\theta in terms of xx and yy. For a point in polar coordinates (ρ,θ)(\rho, \theta), the corresponding Cartesian coordinates are given by x=ρcosθx = \rho\cos\theta and y=ρsinθy = \rho\sin\theta. Substituting ρ=2cosθ\rho = 2\cos\theta into these equations eliminates ρ\rho:

x=2cos2θx = 2\cos^2\theta and y=2cosθsinθy = 2\cos\theta\sin\theta.

Using the double-angle identity cos(2θ)=2cos2θ1\cos(2\theta) = 2\cos^2\theta - 1, we can solve for cos2θ\cos^2\theta:

cos2θ=1+cos(2θ)2\cos^2\theta = \frac{1 + \cos(2\theta)}{2}.

Thus, x=1+cos(2θ)x = 1 + \cos(2\theta).

We apply the identity sin(2θ)=2sinθcosθ\sin(2\theta) = 2\sin\theta\cos\theta to express yy:

y=sin(2θ)y = \sin(2\theta).

Now we are able to express xx and yy without explicit dependence on θ\theta:

(x1)2+y2=1(x - 1)^2 + y^2 = 1.

This is the Cartesian equation of a circle with the center at (1,0)(1, 0) and radius 11.

2. Calculate the distance from the point to the circle's center:
The given point P(2,π3)P(2, \frac{\pi}{3}) in Cartesian coordinates is (1,3)(1, \sqrt{3}) since x=ρcosθ=2cosπ3=212=1x = \rho\cos\theta = 2\cos\frac{\pi}{3} = 2 \cdot \frac{1}{2} = 1 and y=ρsinθ=2sinπ3=232=3y = \rho\sin\theta = 2\sin\frac{\pi}{3} = 2 \cdot \frac{\sqrt{3}}{2} = \sqrt{3}.

The distance dd from point P(1,3)P(1, \sqrt{3}) to the circle's center at (1,0)(1, 0) is:

d=(11)2+(30)2=0+3=3. d = \sqrt{(1 - 1)^2 + (\sqrt{3} - 0)^2} = \sqrt{0 + 3} = \sqrt{3}.

So the distance is 3\boxed{\sqrt{3}}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.