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Question

The inside perimeter of a running track (shown in Fig. 20.24) is 400 m. The length of each of the straight portion is 90 m and the ends are semi-circles. If track is everywhere 14 m wide, find the area of the track. Also, find the length of the outer running track.

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Solution



It is given that the inside perimeter of the running track is 400 m. It means the length of the inner track is 400 m.Let r be the radius of the inner semicircles.Observe: Perimeter of the inner track=Length of two straight portions of 90 m+Length of two semicircles 400=(2×90)+(2×Perimiter of a semicircle)400 = 180+(2×227×r)400-180=(447×r)447×r=220r=220×744=35 m Width of the inner track=2r=2×35=70 mSince the track is 14 m wide at all places, so the width of the outer track: 70+(2×14)=98 m Radius of the outer track semicircles=982=49 mArea of the outer track=(Area of the rectangular portion with sides 90 m and 98 m)+ (2×Area of two semicircles with radius 49 m)=(98×90)+(2×12×227×492)=(8820)+(7546)=16366 m2And, area of the inner track=(Area of the rectangular portion with sides 90 m and 70 m)+ (2×Area of the semicircle with radius 35 m)=(70×90)+(2×12×227×352)=(6300)+(3850)=10150 m2 Area of the running track=Area of the outer track-Area of the inner track=16366-10150=6216 m2And, length of the outer track=(2×length of the straight portion)+(2×perimeter of the semicircles with radius 49 m)=(2×90)+(2×227×49)=180+308=488 m

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