Triangle

### 1.1. Knowledge to remember

**a)** Triangle

The triangle ABC has:

– The three sides are: side AB, side AC, side BC.

– The three vertices are: vertex A, vertex B, vertex C.

The three angles are:

Angle of vertex A, sides AB and AC (referred to as angle A);

Angle of vertex B, sides BA and BC (referred to as angle B);

Angle of vertex C, sides AC and CB (referred to as angle C)

A triangle with three acute angles

A triangle has one obtuse angle and two acute angles

A triangle with one right angle and two acute angles (called a right triangle)

**b)** Bottom and high line

BC is the bottom, AH is the high line corresponding to the BC bottom. The length AH is the height.

### 1.2. Solving textbook exercises on page 86

**Lesson 1 of the textbook page 86:**

Name the three angles and three sides of each triangle below:

*Solution guide:*

The three angles are angle A, angle B, angle C

The three sides are AB, AC, BC

The three angles are: angle D, angle E, angle G

The three edges are: DE, DG, EG

The three angles are: angle M, angle K, angle N

The three sides are MK, MN, and KN.

Lesson 2 Textbook page 86:

Show the base and corresponding height drawn in each of the triangles below:

*Solution guide:*

- Triangle ABC: base is AB, corresponding height is CH.
- Triangle DEG: the bottom is EG, the corresponding high is DK.
- Triangle MPQ: base is PQ, corresponding height is MN.

Lesson 3 Textbook page 86:

Compare the area of:

a) Triangle AED and triangle EDH.

b) Triangle EBC and triangle EHC.

c) Rectangle ABCD and triangle EDC.

*Solution guide:*

a) Area of triangle AED = Area of triangle EDH.

b) Area of triangle EBC = Area of triangle EHC.

c) Area of rectangle ABCD = 2 times area of triangle EDC.

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