A Missing Side
Part of the Triangles section of Coddy's Geometry journey. Lesson 16 of 71.
Now the perimeter is known and one side is not. A triangle has sides 6, 8 and x, and its perimeter is 20. The three sides must add to 20, so you can write an equation and solve it:
Perimeter 20, one side unknown
x + (6 + 8) = 20
x + 14 = 20
x = 6
The two known sides are grouped in brackets so you can add them first. Then one subtraction gives x. Check the answer against the figure, not only the equation. The sides 6, 8 and 6 do add to 20. They also make a triangle, because 6 + 6 > 8. Check this whenever a side comes out of an equation.
Challenge
A triangle has perimeter 24. Two of its sides are 7 and 9.
Find the third side 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