Sharp angle, obtuse angle, flat angle

### 1.1. Knowledge to remember

a) *Acute angle* peak O ; edge OA, OB.

An acute angle is less than a right angle.

b)

*Obtuse angle* peak O ; edge OM, ON.

An obtuse angle is greater than a right angle.

c) *Flat angle* peak O ; edge OC, OD.

Flat angle is equal to two right angles.

### 1.2. Solving Textbook Exercises

**Lesson 1:** Which of the following angles are: right angle, acute angle, obtuse angle, and flat angle?

**Solution guide:**

- Observe the figure and rely on the properties of the angles to determine acute, obtuse, and flat angles.

– Angle of vertex A : side AM, AN and vertex angle D : side DV, DU is acute angle.

– Angle of vertex B : side BP, BQ and vertex angle O : side OG, OH are obtuse angles.

– Angle of vertex C: sides CI, CK are right angles.

– Angle of vertex E: side EX, EY is a flat angle.

**Lesson 2: **In the following triangles

Which triangle has three acute angles?

Which triangle has right angles?

Which triangle has an obtuse angle?

__Solution guide:__

- Observe the figure and rely on the properties of the angles to answer the question of the problem.

A triangle with three acute angles is triangle ABC.

– A triangle with right angles is triangle DEG.

– The triangle with obtuse angle is triangle MNP.

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