The question states:
Find the equation of the following circles: A circle has its centre on the line x + y = 1 and passes through the origin and the point (4,2).
What I did to try to solve this is draw it out and then I tried these two methods which were wrong
- Found a line with (0,0) and (4,2) and intersected it with x + y = 1. Then from there find the equation
- Tried and assumed that the line from (0,0) and (4,2) was the diameter and find the midpoint. Then from there find the equation.
So I was wondering a) why what I did was incorrect b) hints on how to solve it.
In general, I often struggle with a lot of circle coordinate geometry questions and was wondering if you had any tips on how to become better, or see it in a way where I would be able to answer it properly.
Thank you!
Best Answer
Dusting off your geometry book ...
Given any two points on a circle the center lies on to the perpendicular bisector of the chord between them. Here the perpendicular bisector passes through the midpoint $(1/2)((0,0)+(4,2))=(2,1)$. The slope of the chord is clearly $+1/2$ so, by the "negative reciprocal rule" the perpendicular line has slope $-2$. So the center lies on
$y-1=-2(x-2), y=-2x+5$
Since the center also lies on $y=1-x$ we then have for the center:
$1-x=-2x+5, x=4, y =-3$
meaning the center is $(4,-3)$. The radius should now be easy to figure out given that $(4,2)$ is on the circle centered at $(4,-3)$, and the equation of the circle follows.