Associated Topics || Dr. Math Home || Search Dr. Math

### Finding a Point on a Circle

```
Date: 5/28/96 at 16:27:40
From: Anonymous
Subject: Finding a point on a circle

Given a point on the circumference of a circle P(x0,y0), the center
of the circle C(a,b), and the X coordinate of another point on the
circumference of the circle P(x1,y1), how do I find the y1 value?
I already know x0, y0, a, b, and x1, and the Radius R, just not y1.

This is as far as I've gotten:

I've used the formulas x = R cos(t) + a to figure out the value of t:

(x-a)/R = cos(t)

Then I tried to use y = R sin(t) + b to calculate Y, but the value
doesn't appear to be correct.

Joel
```

```
Date: 5/28/96 at 19:45:3
From: Doctor Pete
Subject: Re: Finding a point on a circle

A convenient way to approach this problem is by using Cartesian
coordinates. Since P(x0,y0) is on C, the distance between P(x0,y0) and
the center (a,b) is the radius. Then the equation for C is:

(x-a)^2+(y-b)^2 = (x0-a)^2+(y0-b)^2.

Substituting (x1,y1), we get:

(y1-b)^2 = (x0-a)^2+(y0-b)^2-(x1-a)^2,

and solving for y1, we have

y1 = b +/- sqrt((x0-x1)(x0+x1-2*a)+(y0-b)^2).

Intuitively, this solution makes sense because given some x1, we are
looking for the intersection of the line x = x1 with the circle C,
which in general will have two solutions.  In the cases where the line
is tangent to C and thus has only one solution, it is clear that
y1 = b, corresponding to the plus/minus term in the above expression
being 0.

-Doctor Pete,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
High School Conic Sections/Circles
High School Coordinate Plane Geometry
High School Equations, Graphs, Translations
High School Geometry

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search