A Missing Side

Part of the Triangles section of Coddy's Geometry journey. Lesson 13 of 63.

The perimeter rule works backwards too. If you know the perimeter and two of the sides, the third side is whatever is left: the perimeter minus the two known sides.

$Triangle KLM with perimeter 24 and sides 7 and 9$

Here the perimeter is 24 and two sides are 7 and 9. The third side must make up the total: 24 - 7 - 9, which is 8. Check: 8 + 7 + 9 = 24.

Challenge

△ABC has a perimeter of 30. Find the side AB, 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 EdgeA Missing SideIsosceles Perimeter Sides 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