Due to its length, I’ll division the lesson into three parts. The an initial one will certainly discuss around the position of a line through respect come a circle. The second one will derive a small expression to attain the size of a chord intercepted by a one on a line. The last part will talk about the problems for a heat to end up being a tangent to a circle.

You are watching: What is one relationship between lines and circles

Let’s begin.

Intersection of a Line and a Circle

Given a line and a circle, the line have the right to either

intersect the circle at two different points,touch the circle at just one point, ornot crossing the circle in ~ all.

Drag the slider in the simulation to explore this.

What will recognize whether this line will certainly intersect the circle at two distinctive points, touch the circle in ~ one suggest or will not intersect the one in any kind of point?

There room two ways to think around this.

Method 1

Let the equation that the circle be x2 + y2 = a2, and also that of the heat be y = mx + c.

First, if we try to ‘solve’ the two equations in two unknowns, I acquire a quadratic equation in x, which looks choose this

(1 + m2)x2 + 2cmx + c2 – a2­ = 0

What perform the root of this quadratic (say x1 and x2) represent geometrically?

Now if you recall, once we were talking around equations in general, the x and the y in equations stand for respectively the x and the y coordinates of every points ~ above the curve. Therefore, the root of the vault equation will represent the x collaborates of the clues of intersections that the line and the circle.

In fact, by solving any type of two equations in name: coordinates geometry, we obtain the x (or y) coordinates of the points of intersections that the two curves represented by the equations.

As that now, I’m no interested in the roots themselves, but their nature. Now here’s the link – If we obtain two unique real roots, climate the line will certainly intersect the circle in ~ two unique points. If we achieve two coincident roots, climate the heat is poignant the circle at just one suggest (i.e. 2 coincident points). Finally, if we do not obtain any kind of real root from the developed equation, the line will certainly not touch or intersect the circle.

And just how will you identify that? The nature of root of a quadratic equation is connected to its discriminant. So all we have to do is uncover the discriminant of the quadratic equation, and also check the sign. Positive means intersection in ~ two distinctive points, zero implies tangency, and a negative sign would mean the heat doesn’t crossing or touch the circle.

Here’s a number to summarize what I simply said.


The precise same technique would it is in valid for determining the place of a line in situation of other conic part i.e. Parabola, Ellipse and Hyperbola as well.

But in the instance of circles, geometrical properties will certainly make points a bit easier. This is how.

Method 2

To recognize the position of a line v respect come a circle, all we need to do is find its street from the facility of the circle, and also compare it v its radius. Then,

if the distance is much less than the radius, the line should intersect the circle in ~ two distinct points.if the distance is same to the radius, climate the line will certainly touch the circle.if the distance is better than the radius, the line will certainly lie fully outside the circle.

Here’s one more simulation, whereby you can observe the above conditions.

Hope this was less complicated to understand, compared to the facility quadratic equation.

Lesson Summary

To recognize the position of a line through respect come a circle, we’ll uncover its distance from the facility of the circle. Permit d it is in this distance and be the radius of the circle. Then,

if d if d = r, then the line touches the circle in ~ one point.if d > r, then the line does not intersect/touch the circle.

See more: How To Say 3 40 In Spanish ?

I’ll proceed with a couple of related examples in the next lesson, before moving on come the 2nd part.