Parts of a Circle
The image below shows parts of a circle. The diameter goes from either side of the circumference through the centre. The circumference is the distance outside the circle. A tangent touches the circle at one point on the circumference.
The length of the outside of 2D Shapes is usually called the perimeter, but with circles something funny happens. Can you guess what it is? Here it goes: We call the length of the outside of a circle not a perimeter, but rather instead The Circumference. Why? Who knows! You. Well you do know now 😏. Look let's see.
To work out the length of the circumference you'll need to know the definition and correct spelling for the diameter. We all know how important spelling is in maths so D.I.A.M.E.T.E.R. Got it? Great. So what is it? Oh it's the length from one side of the circle to the other which travels through the center. Take another look:
The diameter of a circle fits around the circumference exactly pi times. This is true for all circles.
What Is The Formula for Area of a Circle?
The image below shows the formula for the area of a circle. The formula for the area of a circle is π x radius squared. The formula for the circumference of a circle is π x diameter.
Circles Question 1
Circles question 1 is work out the area and circumference of the circle below. The area is π x radius x radius. The answer is 153.93 cm². The circumference is π x diameter. The diameter is twice the length of the radius. The answer is 43.98 cm².
Circles Question 2
Circles question 2 works out the area of a semicircle. The radius of the full circle is 43 mm. Since the area of the full circle is 5808.80 mm², the area of the semicircle is 2904.40 mm². The answer is given to 2 decimal places.
Circles Question 3
A photo frame is made from a square sheet of metal. The metal sheet measures 5.5 cm. Work out the total perimeter of the shape.