# Relationship between inscribed angles and central definition

### Inscribed and Central Angles in Circles

Inscribed Angles. An inscribed angle in a circle is formed by two chords that have a common end point on the circle. This common end point is the vertex of the. Inscribed and Central Angles in a Circle: inscribed angle is half of the associated Two chords in a circle, say AC and AB, with a common point A, define an angle BAC. The relationship between the arcs and inscribed angles that we have. CIRCLE DEFINITIONS AND THEOREMS. DEFINITIONS Radius- A segment from the center of the circle to a point on the Inscribed Angle - an angle made from points sitting . the difference of the measures of the intercepted arcs (the.

Let's highlight the arc that they're both subtending. This right here is the arc that they're both going to subtend. So this is my central angle right there, theta. Now if this angle is theta, what's this angle going to be? This angle right here.

Well, this angle is supplementary to theta, so it's minus theta. When you add these two angles together you go degrees around or they kind of form a line. They're supplementary to each other. Now we also know that these three angles are sitting inside of the same triangle. So they must add up to degrees. So we get psi -- this psi plus that psi plus psi plus this angle, which is minus theta plus minus theta.

These three angles must add up to degrees.

### Inscribed angle - Wikipedia

They're the three angles of a triangle. Now we could subtract from both sides. Si plus psi is 2 psi minus theta is equal to 0. Add theta to both sides. You get 2 psi is equal to theta. So we just proved what we set out to prove for the special case where our inscribed angle is defined, where one of the rays, if you want to view these lines as rays, where one of the rays that defines this inscribed angle is along the diameter.

The diameter forms part of that ray. So this is a special case where one edge is sitting on the diameter. So already we could generalize this. So now that we know that if this is 50 that this is going to be degrees and likewise, right? And now this will apply for any time. We could use this notion any time that -- so just using that result we just got, we can now generalize it a little bit, although this won't apply to all inscribed angles. Let's have an inscribed angle that looks like this.

So this situation, the center, you can kind of view it as it's inside of the angle. That's my inscribed angle. And I want to find a relationship between this inscribed angle and the central angle that's subtending to same arc.

So that's my central angle subtending the same arc. Well, you might say, hey, gee, none of these ends or these chords that define this angle, neither of these are diameters, but what we can do is we can draw a diameter.

If the center is within these two chords we can draw a diameter. We can draw a diameter just like that. If we draw a diameter just like that, if we define this angle as psi 1, that angle as psi 2. Clearly psi is the sum of those two angles. And we call this angle theta 1, and this angle theta 2.

So si, which is psi 1 plus psi 2, so psi 1 plus psi 2 is going to be equal to these two things. Psi 1 plus psi 2, this is equal to the first inscribed angle that we want to deal with, just regular si.

What's theta 1 plus theta 2? Well that's just our original theta that we were dealing with. So now we've proved it for a slightly more general case where our center is inside of the two rays that define that angle. Now, we still haven't addressed a slightly harder situation or a more general situation where if this is the center of our circle and I have an inscribed angle where the center isn't sitting inside of the two chords.

Let me draw that. So that's going to be my vertex, and I'll switch colors, so let's say that is one of the chords that defines the angle, just like that. And let's say that is the other chord that defines the angle just like that.

So how do we find the relationship between, let's call, this angle right here, let's call it psi 1. How do we find the relationship between psi 1 and the central angle that subtends this same arc? So when I talk about the same arc, that's that right there. So the central angle that subtends the same arc will look like this. Let's call that theta 1. What we can do is use what we just learned when one side of our inscribed angle is a diameter.

So let's construct that. So let me draw a diameter here. Let's draw a diameter just like that.

## Inscribed Angles Conjectures

Let me call this angle right here, let me call that psi 2. And it is subtending this arc right there -- let me do that in a darker color. It is subtending this arc right there.

So the central angle that subtends that same arc, let me call that theta 2.

## Inscribed and Central Angles in Circles

They share -- the diameter is right there. The diameter is one of the chords that forms the angle. This is exactly what we've been doing in the last video, right? This is an inscribed angle. This common endpoint forms the vertex of the inscribed angle. The other two endpoints define what we call an intercepted arc on the circle. The intercepted arc might be thought of as the part of the circle which is "inside" the inscribed angle.

### Inscribed angle theorem proof (video) | Khan Academy

See the pink part of the circle in the picture above. A central angle is any angle whose vertex is located at the center of a circle.

A central angle necessarily passes through two points on the circle, which in turn divide the circle into two arcs: The minor arc is the smaller of the two arcs, while the major arc is the bigger. We define the arc angle to be the measure of the central angle which intercepts it.

The Inscribed Angle Conjecture I gives the relationship between the measures of an inscribed angle and the intercepted arc angle. It says that the measure of the intercepted arc is twice that of the inscribed angle. The precise statements of the conjectures are given below. Each conjecture has a linked Sketch Pad demonstration to illustrate its truth proof by Geometer's Sketch Pad!

The linked activities sheet also include directions for further "hands on" investigations involving these conjectures, as well as geometric problems which utilize their results.