Sides as Expressions
Part of the Triangles section of Coddy's Geometry journey. Lesson 15 of 63.
When the sides are written with a letter, the perimeter rule gives an equation. Add the three sides, set them equal to the perimeter, and solve.
Here (x + 2) + (x + 5) + (2x + 1) = 36. Collect the x terms and the numbers: 4x + 8 = 36, so 4x = 28 and x = 7. The sides are then 9, 12 and 15, which add up to 36.
On the board, tap a side, pick Perimeter, and the board writes the sum as your line. Finding x is not the end: put it back to get the actual sides when a question asks for them.
Challenge
The sides of △ABC are written with x and its perimeter is 30. Tap a side, pick the rule, and solve the equation the board writes for x.
Then press Check.
Try it yourself
Use the figure to find 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 ReachingThe Range of the Third SideThe FormulaRecap: The Third Side5The Pythagorean Theorem
Squares on the SidesFinding the HypotenusePythagorean TriplesWhen It Is Not WholeRecap: The Hypotenuse3Perimeter
Around the EdgeA Missing SideIsosceles PerimeterSides as ExpressionsRecap: Around the Edge6Finding a Leg
Working BackwardsThe LadderWhich Side Is c?A Leg That Is Not WholeRecap: Finding a Leg