In polar coordinates, find the distance from the point P(2,3π) to the center of the circle given by ρ=2cosθ.
A number or a short expression. Spacing and $ signs are ignored.
Solution
The equation of the circle in polar coordinates is ρ=2cosθ.
1. Convert the polar equation to Cartesian coordinates: To find the center of the circle in Cartesian coordinates, we start by expressing ρ and θ in terms of x and y. For a point in polar coordinates (ρ,θ), the corresponding Cartesian coordinates are given by x=ρcosθ and y=ρsinθ. Substituting ρ=2cosθ into these equations eliminates ρ:
x=2cos2θ and y=2cosθsinθ.
Using the double-angle identity cos(2θ)=2cos2θ−1, we can solve for cos2θ:
cos2θ=21+cos(2θ).
Thus, x=1+cos(2θ).
We apply the identity sin(2θ)=2sinθcosθ to express y:
y=sin(2θ).
Now we are able to express x and y without explicit dependence on θ:
(x−1)2+y2=1.
This is the Cartesian equation of a circle with the center at (1,0) and radius 1.
2. Calculate the distance from the point to the circle's center: The given point P(2,3π) in Cartesian coordinates is (1,3) since x=ρcosθ=2cos3π=2⋅21=1 and y=ρsinθ=2sin3π=2⋅23=3.
The distance d from point P(1,3) to the circle's center at (1,0) is:
d=(1−1)2+(3−0)2=0+3=3.
So the distance is 3.
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