## Book: RD Sharma - Mathematics

### Chapter: 20. Mensuration-I (Area of a Trapezium and a Polygon)

#### Subject: Maths - Class 8th

##### Q. No. 5 of Exercise 20.1

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##### 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.

Perimeter of the inner track = 2× Length of rectangle + perimeter of two semi-circular ends

Perimeter of the inner track = length + length + 2πr

Hence, the radius of inner circle = 35 m

Radius of outer track = radius of inner track + width of the track

Radius of outer track = 35 + 14 = 49m

Length of outer track = 2× Length of rectangle + perimeter of two outer semi-circular ends

Length of outer track = 2× 90 + 2πr

Length of outer track =

Length of outer track =

Area of inner track = area of inner rectangle + area of two inner semi-circles

Area of inner track = length× breadth + πr2

Area of inner track =

Area of inner track = 6300 + 3850

Area of inner track = 10150 m2

Area of outer track = area of outer rectangle + area of two outer semi-circles

Breadth of outer track = 35 + 35 +14 + 14 = 98 m

Area of outer track = length× breadth + πr2

Area of outer track =

Area of outer track = 8820 + 7546

Area of outer track = 16366 m2

Area of path = area of outer track – area of inner track

Area of path = 16366 – 10150 = 6216 m2

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