Base Angles From the Center
Part of the Circles section of Coddy's Geometry journey. Lesson 22 of 62.
In the isosceles triangle OAB, the angle at the center is the apex, and the two base angles are equal. The three angles add to 180°, so take the apex from 180° and split what is left between the two base angles.
Here the central angle AOB is 100°. That leaves 80° for the base angles, so each is 40°. On the board this is one step: tap a base angle and pick Isosceles triangle. The board knows OA and OB are radii and uses the apex.
Challenge
A and B are on circle O, and the angle at the center is marked. Find ∠OAB, then press Check.
Try it yourself
Find the marked part of the figure
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Circles
1Parts of a Circle
Center and RadiusChords and DiametersAll Radii Are EqualDiameter Is Two RadiiRecap: Parts of a Circle4Arcs and Sectors
Arc LengthArea of a SectorFind the Central AngleThe Missing Central AngleRecap: Arcs and Sectors2Circumference
Around the CircleUsing the DiameterRadius From CircumferenceCircumference With LettersRecap: Circumference5Radii Make Isosceles
Two Radii, One TriangleBase Angles From the CenterThe Angle at the CenterIsosceles With LettersRecap: Radii and Triangles8Angles on the Same Arc
Same Arc, Same AngleUsing the Same ArcSpot the Bow TieSame Arc With LettersRecap: Same Arc3Area of a Circle
The Area FormulaArea From the DiameterRadius From the AreaCircumference to AreaRecap: Area of a Circle6Inscribed Angles
Meet the Inscribed AngleHalf the Central AngleTwice the Inscribed AngleThe Big ArcInscribed With LettersRecap: Inscribed Angles