Circle area

### 1.1. Knowledge to remember

*To calculate the area of a circle, multiply the radius by the radius and then multiply by 3.14.*

S = rxrx 3.14

(S is the area of the circle, r is the radius of the circle).

**For example:** Find the area of a circle with radius 2dm.

The area of the circle is:

2 x 2 x 3.14 = 12.56 (dm^{2}).

### 1.2. Solving textbook exercises page 100

**Lesson 1 Textbook page 100**

Calculate the area of a circle with radius rr:

a) r = 5cm

b) r = 0.4dm

c) \(r = \frac{3}{5}m\)

**Solution guide:**

a) The area of the circle is:

5 × 5 × 3.14 = 78.5(cm^{2})

b) The area of the circle is:

0.4 × 0.4 × 3.14 = 0.5024 (dm^{2})

c) \(\frac{3}{5}m\) = 0.6m

The area of the circle is:

0.6 × 0.6 × 3.14 = 1.1304(m^{2})

Lesson 2 Textbook page 100

Find the area of a circle with diameter d:

a) d = 12cm

b) d = 7.2dm

c) \(d = \frac{4}{5}m\)

*Solution guide:*

a) The radius of the circle is:

12 : 2 = 6(cm)

The area of the circle is:

6 × 6 × 3.14 = 113.04(cm^{2})

b) The radius of the circle is:

7.2 : 2 = 3.6 (dm)

The area of the circle is:

3.6 × 3.6 × 3.14 = 40.6944(dm^{2})

c) \(\frac{4}{5}m\) = 0.8m

The radius of the circle is:

0.8 : 2 = 0.4(m)

The area of the circle is:

0.4 × 0.4 × 3.14 = 0.5024(m^{2})

Lesson 3 Textbook page 100

Find the area of a circular table with a radius of 45 cm.

*Solution guide:*

The area of the circular table top is:

45 × 45 × 3.14 = 6358.5(cm^{2})

Answer: 6358.5cm^{2}

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