Join 60,000+ competitive exam aspirants
A cylindrical solid has a radius of 5 cm and a height of 7 cm. A cylindrical cavity of radius 3 cm and depth 2 cm is hollowed out from one of its ends. Find the total surface area (in cm2) of the resulting solid.
126π
107π
82π
132π
132π
The total surface area includes the curved surface area of the main cylinder, the full circular base at the bottom, the ring-shaped top surface, and the inner surface area of the drilled hole.
Cylinder radius r = 5 cm, height h = 7 cm. A cylindrical hole of radius rh = 3 cm and depth d = 2 cm is cut from one end.
TSA=2πr(h)+2πr2+2πrhd
The total surface area includes the curved surface area of the main cylinder, the full circular base at the bottom, the ring-shaped top surface, and the inner surface area of the drilled hole.
Students often forget to include the area of the inner wall of the cylindrical hole or incorrectly subtract the top surface area twice.
Outer CSA Calculation
Calculate the curved surface area of the main cylinder with radius r=5 cm and height h=7 cm: 2πrh=2π(5)(7)=70π.
2πrh=70π
Base and Ring Surface Area
The area of the bottom circular base is πr2=25π. The top surface is a ring with area πr2−πrh2=25π−9π=16π.
Area=25π+16π=41π
Inner Hole Surface Area
The inner wall area (curved surface of the hole) with depth d=2 cm and radius rh=3 cm is 2πrhd=2π(3)(2)=12π.
2πrhd=12π
Total Surface Area
Summing all surfaces: 70π+41π+12π=132π cm².
70π+41π+12π=132π
D is correct because the total surface area of the resultant solid is 132π cm².
This concept is a precursor to understanding volume and surface area changes in engineering drawing and manufacturing processes like drilling or turning.