You will learn to find the perimeter and area

for different shapes and the circumference

of circles.s

s

Perimeter = 4s or

s+s+s+s

Area = s2

l

w

P = 2l+2w or

2(l+w) or

l+w+l+w

A = lwb

h s

P = s1 + s2 + s3

A = ½ bhr

C = 2r = d

A = r2

What is a perimeter?

• Perimeter is the distance all the way around an object.

• The perimeter of a circle is called the circumference.

How To Find Perimeter

Find the length of each side of the object.

Add all the lengths of the sides together.

This total is the perimeter.

Lets’ find the perimeter.

8 + 4 + 8 + 4 = 24.

This rectangle has a perimeter of 24.

How about this perimeter ?

6 + 5 + 5 + 6 + 12 = 34.

This pentagon has a perimeter of 34.

Now lets’ try a circumference:

• c=2πr or c=πd

• r = radius (half the distance across a circle)

• d = diameter (the distance across a circle)

• Note that 2r = d

Find the circumference of a circle with a diameter of 12 cm.

c = πd

• c = π12

• c = 3.14•12

• c = 37.68 cm

12 cm

Now lets’ try a circumference:

Find the circumference of a circle with a radius of 3

meters.

3 m

Lets’ try aother circumference:

• c = 2πr

• c = 2π3• c = 2 • 3.14 • 3

• c = 18.84 m

What if we know the circumference ?

If the circumference of a circle is approximately 50.3 cm, find the radius. (Use 3.14 for π)

C = 2•π•r

50.3 = 6.28r 50.3 / 6.28 = r

8 cm r

Time for area.

• Area of a rectangle• A = bh• A = 4(2)• A = 8 cm²

4 cm

2 cm

Not done yet!!

• Area of a triangle:• A = ½ bh• A = ½ (6)(7)• A = 21 mm²

7 mm

6 mm

What if you are missing aside ?

If the base is 16, and the area is 40, what is the height?

16 in

H = ?40 = ½ (16)h

40 = 8h

40/8 = h

5 in = h

A = ½ bh

Here comes the area of a trapezoid

• A = ½ (b1+b2)h

• A = ½ (6 + 10)(5)• A = ½(16)(5)• A = 40 ft²

b2= 10 ft

b1= 6 ft

5 ft

And yet, here’s another area of a trapezoid

• A = ½ (8 + 12)(3)• A = ½ (20)(3)• A = 30 km²

8 km

12 km

3 km

If the area of the trapezoid is 24, and the height is 4 and base 1

is 8, what is the other

base?

B2 = ? km

8 km

4 km

Let’s find a missing side

24 = ½ (4)(8 + x)24 = 2(8 + x)24 = 16 + 2x

24 – 16 = 2x8 = 2x

4 km = x

A = ½ (h)(b1 + b2)

Just one more, the area of a circle

• A = πr²

• A = 3.14(10)²

• A = 3.14(100)

• A = 314 m²

10 m

OK, one last area of a circle.

• Diameter = 8 in• Radius = 4 in• A = 3.14(4)²• A = 3.14(16)• A = 50.24 in²

8 in

Assignment

1.9 Perimeter, Circumference, and Area

Geometry

Find the perimeter and area of each rectangle. Label each measurement.

1.6 in

3 in

2.

1 yd

12 yd

3.1.65 cm

1.65 cm

4. l = 4.5, w = 1.5, P = ?

Find the missing measure in each formula ifP = 2l + 2w and A = lw.

5. l = 2.2, w = 1.1, A = ?

6. l = 12, A = 30, w = ?

7. A = 3½, w = ½, l = ?

8. P = 13, w = 2.5, l = ?

Find the circumference and area of each circle. Label each measurement.

9.

10.

15 cm

3 m