Question 42

A hollow cylinder of thickness 0.7 cm and height 15 cm is made of iron. If inner radius of cylinder is 3.5 cm, then what is the total surface area (in cm$$^2$$) of the hollow cylinder?

Solution

inner radius,$$r_2=3.5\ cm.$$ and outer radius, $$r_1=\left(0.7+3.5\ \right)cm=4.2\ cm.$$

$$Later\ surface\ \ area\ =\ 2\pi h\left(r_1+r_2\right).$$

Total surface area = $$2\pi h\left(r_1+r_2\right)+2\pi\left(r_1^2-r_2^2\right).$$

= $$2\times\frac{22}{7}15\left(3.5+4.2\right)+2\times\frac{22}{7}\left(4.2^2-3.5^2\right).$$

= $$726+33.88$$

= 759.88 cm.

C is correct choice.

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