Opposite Angles Add to 180
Part of the Circles section of Coddy's Geometry journey. Lesson 43 of 62.
To use the rule, check that all four corners are on the circle, then pair each angle with the one across from it. Knowing one angle of a pair gives the other: take it from 180°.
Here the angle at B is 78°, so the angle at D, across from it, is 180° - 78° = 102°. Tap the unknown angle and pick Cyclic quadrilateral.
Challenge
ABCD has all four corners on circle O. Find the angle at D, 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 Angles9Cyclic Quadrilaterals
Four Corners on a CircleOpposite Angles Add to 180Is It Cyclic?Cyclic With LettersRecap: Cyclic Quadrilaterals