Base Angles Are Equal
Part of the Angles section of Coddy's Geometry journey. Lesson 47 of 62.
An isosceles triangle has at least two equal sides, shown by matching tick marks. The angles opposite those sides are equal. These are the base angles.
Folding an isosceles triangle along the line from the apex to the midpoint of the base makes its two halves coincide. The base angles therefore have equal measures. In the figure, the apex angle is 40°, so 2y + 40 = 180. Subtract 40 and divide by 2 to get y = 70.
Challenge
EasyThe triangle is isosceles with apex angle 50°; each base angle measures x°.
Find x.
Try it yourself
Solve for x
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Angles
1Meet the Angle
Your First AngleNaming AnglesAdding Adjacent AnglesDegrees and the Full TurnAcute, Right, ObtuseRecap: Adjacent Angles4Vertical Angles
Linear PairsVertical Angles Are EqualSolving with VerticalsAll Four AnglesRecap: At the Crossing7Crossing Parallel Lines
Parallel LinesCorresponding AnglesAlternate AnglesCo-interior AnglesWhich Rule Is ItRecap: The Three Pairs10Isosceles Triangles
Base Angles Are EqualFinding the ApexWhich Angles MatchEquilateral AnglesIsosceles at WorkRecap: Twin Corners2Straight Lines and Supplements
Angles on a LineSupplementary AnglesThree Angles on a LineWrite the EquationRecap: Along the Line5Around a Point
Angles Around a PointThe Missing AngleEqual SlicesFour Angles, One PointRecap: The Full Turn3Right Angles and Complements
Complementary AnglesFinding the ComplementPerpendicular LinesEqual Parts of a CornerRecap: The Right Corner6The Angle Bisector
The Angle BisectorBisected ExpressionsBisecting a Straight AngleRecap: Split in Half