Circumscribed and inscribed circles room sketched approximately the circumcenter and also the incenter

In this lesson fine look at circumscribed and also inscribed circles and the unique relationships that form from this geometric ideas.

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Circumscribed circles

When a one circumscribes a triangle, the triangle is inside the circle and the triangle touch the circle with each vertex.


You usage the perpendicular bisectors of each side that the triangle to discover the the facility of the circle that will circumscribe the triangle. So for example, offered ??? riangle GHI???,




The center allude of the circumscribed circle is referred to as the “circumcenter.”

For an acute triangle, the circumcenter is inside the triangle.

For a best triangle, the circumcenter is top top the next opposite appropriate angle.

For an obtuse triangle, the circumcenter is exterior the triangle.

Inscribed circles

When a one inscribes a triangle, the triangle is external of the circle and also the circle touches the sides of the triangle at one allude on each side. The political parties of the triangle room tangent come the circle.

To drawing an inscribed circle inside an isosceles triangle, usage the angle bisectors of each side to find the center of the circle that’s inscribed in the triangle. For example, offered ??? riangle PQR???,

Remember the each side of the triangle is tangent come the circle, for this reason if you draw a radius from the center of the circle to the suggest where the circle touches the leaf of the triangle, the radius will kind a right angle through the leaf of the triangle.

The center suggest of the inscribed circle is dubbed the “incenter.” The incenter will constantly be within the triangle.

Let’s use what us know around these constructions to settle a few problems.

Finding the radius of the circle that circumscribes a trianle


???overlineGP???, ???overlineEP???, and also ???overlineFP??? are the perpendicular bisectors the ???vartriangle ABC???, and also ???AC=24??? units. What is the measure of the radius the the circle that circumscribes ??? riangle ABC????

Point ???P??? is the circumcenter that the circle the circumscribes ??? riangle ABC??? due to the fact that it’s wherein the perpendicular bisectors the the triangle intersect. We can draw ???igcirc P???.

We also know that ???AC=24??? units, and since ???overlineEP??? is a perpendicular bisector the ???overlineAC???, suggest ???E??? is the midpoint. Therefore,


Now us can draw the radius from suggest ???P???, the center of the circle, to suggest ???C???, a allude on that is circumference.

We deserve to use best ??? riangle PEC??? and also the Pythagorean theorem to resolve for the length of radius ???overlinePC???.



You usage the perpendicular bisectors of each side of the triangle to uncover the the center of the one that will certainly circumscribe the triangle.


If ???CQ=2x-7??? and ???CR=x+5???, what is the measure of ???CS???, given that ???overlineXC???, ???overlineYC???, and ???overlineZC??? are angle bisectors that ??? riangle XYZ???.

Because ???overlineXC???, ???overlineYC???, and also ???overlineZC??? are angle bisectors that ??? riangle XYZ???, ???C??? is the incenter the the triangle. The circle with center ???C??? will be tangent to every side the the triangle at the point of intersection.

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???overlineCQ???, ???overlineCR???, and also ???overlineCS??? space all radii of one ???C???, so they’re all equal in length.


We need to uncover the length of a radius. We know ???CQ=2x-7??? and ???CR=x+5???, so






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