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Finding a Side

Part of the Triangles section of Coddy's Geometry journey. Lesson 46 of 63.

The scale factor is not always a whole number, and the unknown can be on either triangle. A proportion handles every case: matching sides have the same ratio.

DE/AB = FD/CA

$Similar triangles ABC and DEF with AB 4, CA 6 and DE 10$

Here AB = 4, DE = 10 and CA = 6, so 10/4 = x/6. Multiply both sides by 6: x = 60/4 = 15. The factor was 2.5. If the unknown were on the small triangle instead, the same proportion would work with the letter on the other side.

Challenge

△ABC ~ △DEF. Find FD, 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 Side Isosceles 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 FactorFinding a SideMatching the Corners Recap: Similar Triangles

12Final Challenges

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