How to calculate the area of a circle. It’s as easy as pi.

How in the world would you find the area of a circle?

Remember, area is always measured in square units.

Remember that the area of a rectangle is length x width because you’re calculating the total number of squares inside of the rectangle. 2 4

That’s fine and dandy, but a circle is not a polygon That’s fine and dandy, but a circle is not a polygon. It does not have straight sides; it has curves.

How are we going to get around these curves?

Imagine chopping up the circle as if it were a pizza.

Now, let’s rearrange our “pizza” into another shape.

PRESTO!

Great Mr Loo! But what in the world is this?

Area of a Circle Discovery Here is what we just did: We labeled everything. Area = base times the height. r r radius Circumference is the distance around the circle. We cut our circle in half making two pieces.

Area of a Circle Discovery Watch what happens when we use a circle with 16 triangles instead of using a circle with 8 triangles. r r Notice that with more triangles the figure begins to really resemble a parallelogram. Area of a parallelogram is base times height.

The area of a circle Use π = 3.14 to find the area of the following circles: 2 cm 10 m A = πr2 A = πr2 = 3.14 × 22 = 3.14 × 52 = 12.56 cm2 = 78.5 m2 23 mm 78 cm A = πr2 A = πr2 = 3.14 × 232 = 3.14 × 392 = 1661.06 mm2 = 4775.94 cm2

Take π = 3.14 Calculate the area of a circle with radius 1.2 cm. ......................................... (b) What is the radius of a circle with an area of 66 cm²? ........................................ Take π = 3.14 Answer The radius is 1.2 cm (a) A = π r ² A = π x r x r A = 3.14 x 1.2 x 1.2 A = 4.5216cm² A = 4.52cm² (2dp) A = π r ² 66 = π x r ² 66 = 3.14 x r ² r =  (66 ÷3.14) r = 4.58cm (2dp)