Test your understanding of the triangle angle rule.

Try to answer the questions before checking the answers at the bottom of the page.

Question 1

Two angles of a triangle are \(40^\circ\) and \(50^\circ\).

Find the third angle.

A) \(80^\circ\)
B) \(90^\circ\)
C) \(100^\circ\)

Question 2

Angles of a triangle are \(90^\circ, 30^\circ,\) and \(x\).

Find \(x\).

A) \(40^\circ\)
B) \(50^\circ\)
C) \(60^\circ\)

Question 3

Two angles of a triangle are equal and the third angle is \(40^\circ\).

Find the value of each equal angle.

A) \(60^\circ\)
B) \(70^\circ\)
C) \(80^\circ\)

Question 4

Angles of a triangle are \(x, x, x\).

Find \(x\).

A) \(60^\circ\)
B) \(90^\circ\)
C) \(120^\circ\)

Question 5

Angles of a triangle are \(70^\circ, 80,^\circ\) and \(x\).

Find \(x\).

A) \(20^\circ\)
B) \(30^\circ\)
C) \(40^\circ\)

Question 6

What is the sum of the interior angles of any triangle?

A) \(90^\circ\)
B) \(180^\circ\)
C) \(360^\circ\)

Question 7

Which fact is used in the proof of the triangle angle sum?

A) A straight angle measures \(180^\circ\)
B) A right angle measures \(180^\circ\)
C) A complete turn measures \(180^\circ\)

Question 8

Why is a line drawn parallel to the base of the triangle in the proof?

A) To create equal alternate interior angles
B) To divide the triangle into two equal parts
C) To find the length of the base

Question 9

The angles of a triangle are:

\(x,\quad 2x,\quad 3x\)

Find \(x\).

A) \(20^\circ\)
B) \(30^\circ\)
C) \(60^\circ\)

Question 10

One angle of a triangle is \(90^\circ\), and the other two angles are equal.

What is the measure of each equal angle?

A) \(30^\circ\)
B) \(45^\circ\)
C) \(60^\circ\)

Answers

1. B – \(90^\circ\)
2. C – \(60^\circ\)
3. B – \(70^\circ\)
4. A – \(60^\circ\)
5. B – \(30^\circ\)
6. B – \(180^\circ\)
7. A – A straight angle measures \(180^\circ\)
8. A – To create equal alternate interior angles
9. B – \(30^\circ\)
   \(x+2x+3x=180^\circ\)
   \(6x=180^\circ\)
   \(x=30^\circ\)
10. B – \(45^\circ\)
    \(90^\circ+x+x=180^\circ\)
    \(2x=90^\circ\)
    \(x=45^\circ\)

Practice

• More Examples
• Take the Quiz (Current page)

Continue Learning

1. Sum of Angles of a Triangle is 180° (Proof)
2. Find the Missing Angle of a Triangle

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