Areas of Combinations of Plane Figures
Solution : Area of the square lawn ABCD = 56 × 56 $m^2$ Let OA = OB = $x$ metres So, $x^2$ + $x^2$ = 562 or, 2$x^2$ = 56 × 56 or, $x^2$ = 28 × 56 (2) Now, area of sector OAB = $90/360$ ×π$x^2$= $1/4$ ×π$x^2$ =$1/4$ ×$22/7$ × 28 × 56 Also, area of ∆ OAB = $1/4$ × 56 × 56 $m^2$ ( Since ∠AOB = 90°) So, area of flower bed AB = $1/4$ ×$22/7$ × 28 × 56 - $1/4$ × 56 × 56 = $1/4$ × 28 × 56 × $8/7$ $m^2$ Similarly, area of the other flower bed =$1/4$ × 28 × 56 × $8/7$ $m^2$ Therefore, total area = Area of the square lawn ABCD + 2 × area of flower bed AB = 56 × 56 + 2 × ($1/4$ × 28 × 56 × $8/7$) $m^2$ = 4032 $m^2$Alternative Solution : Total area = Area of sector OAB + Area of sector ODC + Area of ∆ OAD + Area of ∆ OBC =$90/360$ × $22/7$ × 28 ×56 + $90/360$ × $22/7$ × 28 ×56 + $1/4$ ×56 ×56 + $1/4$ ×56 ×56 $m^2$ = 56 × 72 $m^2$ = 4032 $m^2$Example: Find the area of the shaded region in Fig. , where ABCD is a square of side 14 cm.Solution : Area of square ABCD = 14 × 14 $cm^2$ = 196 $cm^2$ Diameter of each circle = $14/2$ $cm$ = 7 $cm$ So, radius of each circle = $7/2$ $cm$ So, area of one circle = $πr^2$ =$22/7$×$7/2$×$7/2$ $cm^2$ =$77/2$ $cm^2$ Therefore, area of the four circles = 4× $77/2$ $cm^2$ = 154 $cm^2$ Hence, area of the shaded region = Area of square ABCD - area of the four circles =(196 – 154) $cm^2$ = 42 $cm^2$.Example: Find the area of the shaded design , where ABCD is a square of side 10 cm and semicircles are drawn with each side of the square as diameter. (Use π = 3.14)Solution : Let us mark the four unshaded regions as I, II, III and IV (see Fig. ). Area of I + Area of III = Area of ABCD – Areas of two semicircles of each of radius 5 cm =$({10 ×10 }-2 × 1/2 × π×5^2 )$ $cm^2$ = 21.5 $cm^2$ Similarly, Area of II + Area of IV = 21.5 $cm^2$ So, area of the shaded design = Area of ABCD – Area of (I + II + III + IV) = (100 – 2 × 21.5) $cm^2$ = (100 – 43) $cm^2$ = 57 $cm^2$