Isosceles Perimeter
Part of the Triangles section of Coddy's Geometry journey. Lesson 17 of 71.
An isosceles triangle has two equal sides, so its perimeter counts one length twice. Suppose the equal sides are x, the base is 10 and the perimeter is 26. Write the equation and solve it:
Two sides of x and a base of 10
2x + 10 = 26
2x = 16
x = 8
The term 2x stands for the two equal sides together. You could also write x + x + 10 = 26, which says the same thing one step earlier. Either way the equal sides are 8, so the triangle has sides 8, 8 and 10. The base is the different side, and it appears once in the equation.
Challenge
An isosceles triangle has perimeter 30. Its base is 8 and its two equal sides are 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 Triangles
1Sides and Names
Three Sides, Three CornersSorting by SidesNaming by AnglesTicks into EquationsThe Longest SideRecap: Sorting Triangles2The Triangle Inequality
Can Three Sticks Meet?Exactly ReachingHow Short Can It Be?How Long Can It Be?The Range of the Third SideThe FormulaWhole-Number SidesRecap: The Third Side5The Hypotenuse
Squares on the SidesFinding the HypotenusePythagorean TriplesWhen It Is Not WholeSimplifying the RootRecap: The Hypotenuse8Special Right Triangles
The 45-45-90 TriangleLegs from the HypotenuseThe 30-60-90 TriangleAll Three SidesWhich Triangle Is It?Recap: Special Triangles11Mixing Area and Pythagoras
Height of an IsoscelesArea from the HeightThe Equilateral TriangleRight Triangle PerimeterRecap: Two Tools3Perimeter
Around the EdgeA Missing SideIsosceles PerimeterEquilateral PerimeterSides as ExpressionsWrite the PerimeterRecap: Around the Edge6Finding a Leg
Working BackwardsThe LadderWhich Side Is c?A Leg That Is Not WholeRecap: Finding a Leg