Area of a circle Lesson 4.5 February 12th, 2012

It’s quiz time!!! #1 What is the formula for finding the area of a triangle?

#2 What is the relationship between parallelograms and triangles?

What is the area of the triangles? #3 What is the area of the triangles? 6 cm 6 cm

#4 What does congruent mean?

#5 Find the area of a triangle with a base of 7 and a height of 4.

#6 Find the area of this triangle. 5.1 cm 7.2 cm

#7 Find the area of the triangles. 1 cm 3 cm 3 cm 1 cm 7 cm

#8 What is the diameter of this circle? 5.4 mm

#9 What is the area of this parallelogram? 14 m 20.2 m

#10 What is the circumference of a circle with a radius of 3.6 ft?

b x h ÷ 2

#2 There are two congruent triangles in one parallelogram. OR A parallelogram divided in two equals two congruent triangles.

#3 Area of parallelogram A = b x h A = 6 x 6 A = 12 cm Area of triangles A = 12 ÷ 2 A = 6 cm

#4 Two shapes that are EXACTLY the same, but, they are not in the same orientation. OR Two shapes that are the same, but facing a different way.

#5 A = b x h ÷ 2 A = 7 x 4 = 28 28 ÷ 2 = A = 14 cm

#6 A = b x h ÷ 2 A = 7.2 x 5.1 ÷ 2 A = 18.36 cm

#7 1 x 3 = 3 ÷ 2 1.5 cm

#8 D = 5.4 x 2 D = 10.8 mm

A = b x h A = 20.2 x 14 = 282.8 m A = 282.8 m

C = 2Лr C = 2 x 3.14 x 3.6 C = 22.6 ft

Toilet paper tube... If I cut a circle in half...I get a rectangle! Each of the shorter sides of the rectangle would be equal to the radius. The sum of the two longer sides is equal to one half the circle’s circumference.

Area of a rectangle = b x h Height = r Base = Лr

Therefore, The area of the rectangle is Лr x r = Лr squared. So, the formula for finding the area of a circle is A = Лr squared. You can use this formula anytime you know the radius.

Find the area of this circle.

Find the area of this circle.