Lesson

[Definition]

Isosceles Triangles

An isosceles triangle has two equal sides and two equal base angles

Example

Match the numbered statements in Column X with the letters allocated to the diagrams in Column Y by classifying according to sides or angles.
Explain your answers.

Column X Column Y
1

An acute triangle has 3 acute angles

A
2

A right triangle has 1 right angle

B
3

An obtuse triangle has 1 obtuse angle

C
4

An equilateral triangle has 3 congruent sides

D
5

An isosceles triangle has at least 2 congruent sides

E
6

A scalene triangle has no sides of equal length

F

Solution

1-E

All angles are less than 90°

2-A

One angles is equal to 90°

3-B

One angle is greater than 90°

4-C

All sides are of equal length

5-F

Two sides are of equal length

6-D

None of the sides are equal length

Question

A triangle’s largest angle is 2.5 times larger than the smallest angle. If the third angle is 40°, what type of triangle is it?

Explanation

Given the information, we can create an equation and solve.

2.5x + x +40 = 180

3.5x = 140

x = 40

Therefore, the angles are 100°, 40°, 40° and the triangle is isosceles.

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