The angle is the distance between two lines that meet at point and measured in degree. The angle formed by the half open book is of 90 degree.

What is an angle?

An angle is a geometric figure formed when two lines or rays meet at a particular point called the vertex. The lines or rays that form the angle are known as the sides of the angle. Angles are measured in degrees, which is a unit of measurement for angles.

Types of angles:

There are various types of angles based on their measurements. Let's discuss them below:

1. Right angle:
- A right angle is formed when two lines or rays meet to form a 90-degree angle.
- It is denoted by the symbol "⦟".

2. Acute angle:
- An acute angle is formed when two lines or rays meet to form an angle less than 90 degrees.
- It is denoted by the symbol "⦞".

3. Obtuse angle:
- An obtuse angle is formed when two lines or rays meet to form an angle greater than 90 degrees but less than 180 degrees.
- It is denoted by the symbol "⦡".

4. Straight angle:
- A straight angle is formed when two lines or rays meet to form a 180-degree angle.
- It is denoted by the symbol "⦣".

5. Reflex angle:
- A reflex angle is formed when two lines or rays meet to form an angle greater than 180 degrees but less than 360 degrees.
- It is denoted by the symbol "⦦".

The angle of an open book:

When a book is open halfway, it forms a right angle. A right angle measures exactly 90 degrees. Therefore, the correct answer for the angle of an open book is option A. 90 degrees.

To summarize, an angle is formed when two lines or rays meet at a point. There are different types of angles, including right angles, acute angles, obtuse angles, straight angles, and reflex angles. When a book is open halfway, it forms a right angle, which measures 90 degrees.