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
1E All angles are less than 90° 
2A One angles is equal to 90° 
3B One angle is greater than 90° 
4C All sides are of equal length 
5F Two sides are of equal length 
6D 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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