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Isosceles Perimeter

Part of the Triangles section of Coddy's Geometry journey. Lesson 14 of 63.

In an isosceles triangle the two ticked sides are equal, so the perimeter is two equal sides plus the base. If you know the perimeter and the base, take the base away and halve what is left.

$Isosceles triangle DEF with base 6 and perimeter 20$

Here the base is 6 and the perimeter is 20. The two equal sides share 20 - 6 = 14, so each is 7. You use two facts at once: the perimeter and the ticks. On the stage pick Perimeter; the board folds in the ticks for you.

An equilateral triangle is the extreme case: all three sides are equal, so each side is the perimeter divided by 3.

Challenge

△ABC is isosceles: the two ticked sides are equal. Its perimeter is 32. Find BC, then press Check.

Try it yourself

$Find the marked part of the figure$

Find the marked part of the figure

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Triangles

2The Triangle Inequality

Can Three Sticks Meet?Exactly Reaching The Range of the Third Side The Formula Recap: The Third Side

5The Pythagorean Theorem

Squares on the Sides Finding the Hypotenuse Pythagorean Triples When It Is Not Whole Recap: The Hypotenuse

8Special Right Triangles

The 45-45-90 Triangle Legs from the Hypotenuse The 30-60-90 Triangle All Three Sides Which Triangle Is It?Recap: Special Triangles

11Two Tools at Once

Height of an Isosceles Area from the Height The Equilateral Triangle Right Triangle Perimeter Recap: Two Tools

3Perimeter

Around the Edge A Missing SideIsosceles PerimeterSides as Expressions Recap: Around the Edge

6Finding a Leg

Working Backwards The Ladder Which Side Is c?A Leg That Is Not Whole Recap: Finding a Leg

9Similar Triangles

Same Shape, Different Size The Scale Factor Finding a Side Matching the Corners Recap: Similar Triangles

12Final Challenges

Final: Three Sticks Final: The Fence Final: The Ladder Final: The Shadow Final: Master of Triangles