# Find

i) the lateral or curved surface area of a closed cylindrical petrol storage tank that is 4.2 m in diameter and 4.5 m high.

ii) how much steel was actually used, if 1/12 of the steel actually used was wasted in making the tank.

**Solution:**

The curved surface area of a right circular cylinder of base radius 'r' and height 'h' is 2πrh and its total surface area is 2πr(r + h).

Diameter of the tank, d = 4.2 m

Radius of the tank, r = d/2 = 4.2/2 m = 2.1 m

Height of the tank, h = 4.5 m

i) Lateral or curved surface area of the tank = 2πrh

= 2 × 22/7 × 2.1 m × 4.5 m

= 59.4 m²

ii) Total surface area of the tank = 2πr (h + r )

= 2 × 22/7 × 2.1 m × (4.5 m + 2.1 m)

= 2 × 22/7 × 2.1 m × 6.6 m

= 87.12 m²

Let the amount of steel required to make the tank be 'x'.

Amount of steel required - Amount of steel wasted = Total surface area of the tank

x - x/12 = 87.12 m² [Since, 1/12 of the steel was wasted]

11x/12 = 87.12 m²

x = 12/11 × 87.12 m²

x = 95.04 m²

Thus, Curve surface area = 59.4m², Steel actually used to make the tank = 95.04m²

**Video Solution:**

## Find i) The lateral or curved surface area of a closed cylindrical petrol storage tank that is 4.2 m in diameter and 4.5 m high. ii) How much steel was actually used, if 1/12 of the steel actually used was wasted in making the tank?

### Class 9 Maths NCERT Solutions - Chapter 13 Exercise 13.2 Question 9:

**Summary:**

For a closed cylindrical petrol storage tank that is 4.2 m in diameter and 4.5 m high, we have found that the curved surface area = 59.4m² and the amount of steel actually used in making the tank = 95.04 m².