Menu

The Longest Side

Part of the Triangles section of Coddy's Geometry journey. Lesson 6 of 60.

Open a pair of scissors wider, and the tips move further apart. A triangle works the same way. The biggest angle is across from the longest side. The smallest angle is across from the shortest side. The side across from ∠A is the side that does not touch A. That is BC.

The biggest angle also gives the triangle its name:

Acute: every angle is less than 90°.
Right: one angle is 90°.
Obtuse: one angle is more than 90°.

$Triangle PQR with an angle of 35 degrees at P and 25 degrees at Q$

The three angles of a triangle add up to 180°. So here ∠R is $180-35-25 = 120$ degrees. The triangle is obtuse, and its longest side is PQ.

Challenge

△ABC shows two of its angles. Work out the third angle. Then tap the longest side. Use the angles, not how long the sides look. Then press Check.

Try it yourself

$Find the right part on the figure$

Find the right part on the figure

All lessons in Triangles

1The Triangle Inequality

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

4Area of a Triangle

Half the Rectangle Height Is Not a Side Right Triangle Area Finding the Height Any Side Can Be the Base Recap: Area

7Is It Right?

The Converse Testing a Triangle Acute or Obtuse?Sorting by the Test Recap: Right or Not

10Scale in the World

Shadows Across the River Nested Triangles A Side with an Expression Recap: Scale in the World

2Sides and Names

Sorting by SidesThe Longest SideTicks into Equations Recap: Sorting Triangles

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