**Table of Contents**

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**Introduction To Angles **

Do you know how an angle is formed?

When two rays meet at a vertex, an angle is formed.

An angle is represented by the symbol \(\angle\) and is measured in degrees \(^\circ\)

There are different types of angles.

They can be acute, obtuse, reflex, straight, right and whole.

We will learn about acute angles in this topic.

**What is an Acute Angle?**

An **acute angle** is an angle whose measure is greater than \({0^\circ}\) and less than \({90^\circ}\).

Here are a few examples of acute angles.

**Definition of an Acute Angle**

The definition of an acute angle states that an angle whose measure is greater than 0° and less than 90° is an acute angle.

Look at the simulation below.

Drag point B to see the acute angles that can be formed here.

**Examples of Acute Angles**

We know that angles measuring greater than 0° and less than 90° are called acute angles in geometry.

Therefore, \(45^\circ \), \(5^\circ \), \(18^\circ \), \(49^\circ \), \(89^\circ \) are all examples of acute angles.

Here are some real-life examples of acute angles.

- A slice of watermelon
- An intersection of a crossroad
- Some instances of the angles formed between the hour's hand and the minute's hand of the clock
- Angle between the arms of a pair of scissors
- The beak of a bird when it is open
- An angle formed when a crocodile mouth is open

Look around and you will be observe other examples of acute angles.

**Properties of Acute Triangles**

An acute triangle has all its angles less than 90°

When all three angles of a triangle are 60°, it forms a special triangle called an **equilateral triangle**.

Acute triangles can be classified as acute scalene triangles, acute isosceles triangles and equilateral triangles.

You can go through types of triangles for more information on these triangles.

**Acute Angle Formula**

Just like the Pythagoras theorem for right triangles, we have a triangle inequality for acute triangles.

It states that the sum of the squares of the two sides is greater than the square of the greatest side.

In \(\Delta ABC \), the sides measure \(a,b,c \) such that \(c\) is the largest side, then:

\(a^2 + b^2 > c^2\) |

Conversely, in a triangle, if \(a^2 + b^2 >c^2\), the triangle is an acute triangle.

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**Solved Examples **

Example 1 |

Here is a small activity for you to try.

When you click on Generate New Angle, a random angle is generated.

Measure the angle formed using a protractor and identify if it is an acute angle.

**Solution: **

All the angles formed measure less than 90°

Hence, they are all acute angles.

All the formed angles are acute angles. |

Example 2 |

From the following options, choose the angles which can be classified as acute angles.

**Solution: **

Option \(a\) and option \(d\) are less than \(90^\circ \)

Hence, they are acute angles.

Option (a) and option (d) are acute angles. |

Example 3 |

At what time, in the clock shown below, an acute angle is formed?

**Solution: **

We can observe that at 10 o'clock and 11 o'clock, the angle formed between the hour hand and the minute hand is an acute angle.

\(\therefore\) An acute angle is formed at 11 o'clock and 10 o'clock. |

Example 4 |

Is the following triangle acute?

**Solution: **

All the 3 angles should measure less than 90° in an acute triangle.

From the figure, we can see that two angles measure 45° each.

\(\angle Y\) is a right angle.

Hence, this is not an acute triangle.

The given triangle is not an acute triangle. |

Example 5 |

Can an acute triangle have sides that measure 4 cm, 5 cm, and 8 cm?

**Solution: **

In an acute triangle, the sum of the squares of the two sides should be greater than the square of the greatest side.

i.e \(a^2 + b^2 >c^2\) where c is the largest side.

Let a = 4 cm

b= 5 cm

c = 8 cm (largest side )

\(a^2 = 16\)

\(b^2 = 25\)

\(c^2 = 64\)

\(a^2 +b^2= 16 +25 = 41\)

Since \( 41 < 64 \)

\( \implies a^2 + b^2 \) is less than \(c^2\)

Hence, the given measures do not form an acute triangle.

4 cm, 5 cm and 8 cm cannot form an acute triangle. |

- How many acute angles are there in the given images?

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**Practice Questions**

**Here are a few activities for you to practice. **

**Select/Type your answer and click the "Check Answer" button to see the result**

- Angles are measured in degrees (°) and are represented with \(\angle\)

Example: \(\angle ABC = 34^\circ\) - Acute angles in Geometry refer to angles that measure greater than 0° and less than 90°.
- We use a protractor to measure angles.

**Maths Olympiad Sample Papers**

IMO (International Maths Olympiad) is a competitive exam in Mathematics conducted annually for school students. It encourages children to develop their math solving skills from a competition perspective.

You can download the FREE grade-wise sample papers from below:

- IMO Sample Paper Class 1
- IMO Sample Paper Class 2
- IMO Sample Paper Class 3
- IMO Sample Paper Class 4
- IMO Sample Paper Class 5
- IMO Sample Paper Class 6
- IMO Sample Paper Class 7
- IMO Sample Paper Class 8
- IMO Sample Paper Class 9
- IMO Sample Paper Class 10

To know more about the Maths Olympiad you can **click here**

**Frequently Asked Questions(FAQs) **

## 1. What is the degree measure (number) of an acute angle?

An angle whose measure is greater than 0° and less than 90° is called an acute angle.

## 2. What is an acute angle triangle?

A triangle where all three vertex angles measure less than 90° is called an acute angle triangle.

## 3. What is an acute angle parallelogram?

In a parallelogram, the consecutive angles are supplementary and opposite angles are equal.

Hence, there can be two acute angles which are opposite to each other.

The other two angles are obtuse angles.

A rectangle/square is a special case of a parallelogram where all angles are right angles.