Perimeter of triangles, quadrilaterals

### 1.1. Knowledge to remember

How to calculate the perimeter of a triangle and the perimeter of a quadrilateral.

### 1.2. Math forms

**Form 1: Find the perimeter of a triangle.**

To calculate the perimeter of a triangle, find the sum of the lengths of the three sides of the triangle

**Form 2: Find the perimeter of a quadrilateral.**

To find the perimeter of a quadrilateral, we need to find the sum of the lengths of the sides of the quadrilateral.

**Form 3: Compare the length of the bend with the perimeter of triangles, quadrilaterals.**

– Calculate the length of the bend, the perimeter of the triangle, quadrilateral.

– Convert the measurement units to the same unit (if necessary) and then compare.

### 1.3. Solving Textbook Exercises page 130

**Lesson 1**

Find the perimeter of a triangle whose side lengths are:

a) 7cm, 10cm and 13cm.

b) 20dm, 30dm and 40dm.

c) 8cm, 12cm and 7cm.

Sample: Perimeter of a triangle is:

7 + 10 + 13 = 30 (cm)

Answer: 30cm.

__Solution method__

The perimeter of a triangle is equal to the sum of the lengths of its sides.

Present the problem according to the model.

__Solution guide__

b) Perimeter of the triangle is:

20 + 30 + 40 = 90 (dm)

Answer: 90dm.

c) Perimeter of the triangle is:

8 + 12 + 7 = 27 (cm)

Answer: 27cm.

**Lesson 2**

Find the perimeter of a quadrilateral whose side lengths are:

a) 3dm, 4dm, 5dm and 6dm.

b) 10cm, 20cm, 10cm and 20cm.

__Solution method__

The perimeter of a quadrilateral is equal to the sum of the lengths of its four sides.

__Solution guide__

a) Perimeter of the quadrilateral is:

3 + 4 + 5 + 6 = 18 (dm)

Answer: 18dm.

b) The perimeter of the quadrilateral is:

10 + 20 + 10 + 20 = 60 (cm)

Answer: 60cm.

**lesson 3**

a) Measure and record the lengths of the sides of triangle ABC.

b) Calculate the perimeter of triangle ABC.

__Solution method__

– Use a straightedge with centimeter divisions to measure the lengths of the sides.

The perimeter of a triangle is equal to the sum of the lengths of its 3 sides.

__Solution guide__

a) Students measure, each side is 3cm and record:

b) Perimeter of triangle ABC is:

3 + 3 + 3 = 9 (cm)

Answer: 9cm.

### 1.4. Solving Textbook Exercises page 131

**Lesson 1**

a) A curved line consists of three straight segments:

b) A triangle:

c) A quadrilateral.

__Solution method__

Use a straightedge to connect the given points to form the required shapes.

__Solution guide__

Multiple ways can be concatenated, such as the following:

**Lesson 2**

Find the perimeter of triangle ABC, given the lengths of the sides:

AB = 2cm, BC = 5cm, AC = 4cm.

__Solution method__

The perimeter of a triangle is equal to the sum of the lengths of its sides (in the same units).

__Solution guide__

The perimeter of triangle ABC is:

2 + 4 + 5 = 11 (cm)

Answer: 11cm.

**lesson 3**

The quadrilateral DEGH has side lengths DE = 3cm, EG = 5cm, GH = 6cm, DH = 4cm. Calculate the perimeter of that quadrilateral.

__Solution method__

The perimeter of a quadrilateral is equal to the sum of the lengths of its 4 sides (same units).

__Solution guide__

Perimeter of quadrilateral DEGH is:

3 + 5 + 6 + 4 = 18 (cm)

Answer: 18cm.

**Lesson 4**

a) Calculate the length of the bend ABCDE.

b) Calculate the perimeter of quadrilateral ABCD.

__Solution method__

– The length of the bend is equal to the sum of the lengths of the sides AB; BC; CD and DE.

The perimeter of a quadrilateral is equal to the sum of the lengths of its four sides.

__Solution guide__

a) The length of the bend ABCDE is:

3 + 3 + 3 + 3 = 12 (cm)

Answer: 12 cm

b) Perimeter of quadrilateral ABCD is:

3 + 3 + 3 + 3= 12 (cm)

Answer: 12 cm.

**Attention:**

In addition, you can write the calculation 3 + 3 + 3 + 3 into 3 x 4 to find the solution to the problem.

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