Proving Equidistance of Intersection Points on an Archimedean Spiral
Understanding the Problem
We're considering the Archimedean spiral defined by the polar equation r = θ for θ > 0. The task is to prove that consecutive intersection points of this spiral with any ray originating from the origin are equidistant.
Solution
1. Parametric Representation of the Ray:
Let the ray be defined by the angle φ.
Any point on this ray can be represented in polar coordinates as (r, φ), where r is the distance from the origin.
2. Intersection Points:
To find the intersection points of the spiral and the ray, we equate their polar equations:
θ = r
Substituting r with r = ρcos(φ - θ) (polar coordinate conversion to Cartesian), we get:
θ = ρcos(φ - θ)
3. Finding Consecutive Intersection Points:
Let θ₁ and θ₂ be the angles corresponding to two consecutive intersection points.
Then, we have:
θ₁ = ρcos(φ - θ₁)
θ₂ = ρcos(φ - θ₂)
4. Calculating the Distance Between Intersection Points:
The distance between two points in polar coordinates is given by:
d = sqrt(r₁² + r₂² - 2r₁r₂cos(θ₂ - θ₁))
Substituting r₁ = θ₁ and r₂ = θ₂, we get:
d = sqrt(θ₁² + θ₂² - 2θ₁θ₂cos(θ₂ - θ₁))
5. Simplifying the Distance:
Using the trigonometric identity cos(A - B) = cosAcosB + sinAsinB, we can simplify the expression for d:
d = sqrt((θ₂ - θ₁)²)
d = |θ₂ - θ₁|
6. Conclusion:
Since θ₂ and θ₁ represent consecutive intersection points, |θ₂ - θ₁| is a constant (equal to 2π for a full rotation).
Therefore, d = 2π is also a constant.
The Distance
The distance between consecutive intersection points of the Archimedean spiral with any ray from the origin is 2π.
This proves that the consecutive intersection points are equidistant.