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Circumference of a Circle Math 10-3 Ch.3 Measurement Properties of Circles… Yesterday we talked about the perimeter of various 3 sided and 4 sided shapes. How do we find the perimeter of a circle? First, let’s review the properties of the circle: *All the points on a circle are equidistant (the same distance) from the CENTER of the circle. Properties of Circles… *A line that passes through the center of a circle and touches the edge of the circle on both sides is called the diameter. Properties of Circles… *A line that starts at the center of the circle and touches an outside edge is called the radius. Diameter and Radius… *The radius can be calculated by dividing the diameter by 2 r d 2 *The diameter can be calculated by multiplying the radius by 2 d 2r *The circumference of a circle is the perimeter of the circle. It can be calculated with the formula : C d Where ◦ C = circumference (perimeter) ◦ = “pi” a constant that is 3.14159…. ◦ d = diameter Ex1. What is the circumference of a circle with a diameter of 8 cm? C=? d= 8cm C d C = x 8 C = 3.14 x 8 = 25.12 cm *note: we will use the estimation of 3.14 for pi in this course Ex2. What is the circumference of a circle with a radius of 2.5 mm? C=? d= ? r = 2.5 mm *first we must find the diameter of the circle. d=2xr d= 2 x 2.5 mm = 5 mm Ex2. What is the circumference of a circle with a radius of 2.5 mm? C d C= x5 = 3.14 x 5 = 15.7 mm Ex3. The circumference of a circle is 52cm. What is the diameter? C = 52 cm d=? *We must perform opposite operations (algebra!) to calculate the diameter! First, fill in the formula with what we know: 52 cm = 3.14 x d What is opposite of multiplying by 3.14? Dividing by 3.14 on the other side! 52 cm 3.14 = d d = ~16.56 cm C d