Area of a Circle Mrs. Cronnelly

Why don’t we have just one formula for area? Why is it that the area of a square or a rectangle is base x height but the area of a circle is A =  r²? Wouldn’t it make sense to have just one formula for area? Well actually, we do! It is base x height!

Here is a step by step process to help us see this more clearly! If we cut a pie that has A diameter of 9 inches into eight slices and arrange them into a parallelogram Like this! 9 in

We now have a parallelogram! height 4.5 in r base 14.13 in A = b x h A = 14.13 x 4.5 A = 63.585 in² How close are we? A =  r² A = 3.14 x 4.5² A = 63.617 in²

Let’s see how well you can find the Circumference of a circle!

(Round to the nearest tenth) C= 12.6 cm C= 72.2 mm C= 56.5 m C= 364.2 cm

Now let’s practice how to find the area of a circle!

(Round to the nearest tenth) A= 254.3 m² A= 10,563 cm² A= 12.6 cm² A= 415.3 mm²

Mix it up!!!!! A = 210.665 cm² 52.82 cm Area Distance around (two halves make a whole circle) Area Distance around Find the area of the circle Find Circumference of a circle A=132.665 cm² 40.82 cm Find the area of a rectangle Find Perimeter of rectangle (be careful!) A = 78 cm² 12 cm A = 210.665 cm² 52.82 cm

Radius = 3.8 cm Diameter =7.6 cm Area = 45.3 cm² (Round to the nearest tenth) Radius = 3.8 cm Diameter =7.6 cm Area = 45.3 cm²

Radius = 3.5 ft Area = 38.465 ft² Diameter = 7 ft (Round to the nearest tenth) Radius = 3.5 ft Area = 38.465 ft² Diameter = 7 ft Circumference = 21.98 ft