Calculating The Area Of a Circle So far In this topic we have come up with a formula to calculate the circumference of a circle : C = πD Now we need a formula to calculate the area of a circle!

The Formula! Area of a circle = π x radius 2 OR, in short, A = π r 2 r

Examples : 1.Calculate the areas of the following circles : (a) 4cm (b) 17.5cm A = π r 2 A = π x 4 2 A = π x 16 A = (to 2d.p.) A = π r 2 A = π x A = π x A = (to 2d.p.)

A slightly more difficult example! 2. The brim of Professor McGonagalls witch’s hat is made by cutting a small circle out of the middle of a larger circle. (a)Calculate the area of the larger circle (b)Calculate the area of the smaller circle (c)By subtracting, calculate the area of the brim 15cm 7cm

Area of Large Circle :A = π r 2 A= π x 15 2 A = π x 225 A = (to 2d.p.) Area of Inner Circle :A = π r 2 A = π x 7 2 A = π x 49 A = Area Of Brim =Area of Large Circle – Area of Inner Circle = – = cm 2 15cm 7cm

24cm Find the shaded area. Square A=24  24 = 576cm² 1 circle A=  r² r =6cm =   6  6 A= … Area of 4 circles =  4 = = 452cm² Shaded area = 576cm² - 452cm² = 124cm²

MIXED QUESTIONS 4cm 10cm Find the perimeter and the area of this shape. Perimeter Area C =  d =   10 Perimeter = = 39.4cm A=  r² =   5  5 = 78.5cm² CircleRectangle A= 4  10 = 31.4cm = 40cm² Total area = = 118.5cm²

60cm 40cm Find the shaded area. Shaded area area of rectangle – area of semi-circle A = 60  40 = 2400cm² A=  r²  2 =   30  30  2 r =30cm = … = 1414cm² Shaded area = = 986cm²