Let us find out the surface areas
of all these objects !
Cuboid
Cube
Cylinder
Cone
Sphere
Surface area of a cuboid
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A bundle of many sheets of
paper makes up a cuboid.
We have used six
rectangular pieces to cover
the complete outer surface
of the cuboid.
Area of 6 rectangles =
2(lxb) + 2(bxh) + 2(lxh)
Surface area of a cuboid
= 2 (lb + bh + hl)
Surface area of a cube
A cuboid, whose length,
breadth and height are all
equal, it is called a Cube. If
each edge of the cube is a,
then the surface area of this
cube would be:
2(axa+axa+axa),i.e.,6a², giving
us –
Surface area of a cube =6a²
where a is the edge of the cube.
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Surface area of a right circular
cylinder
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Take a
number of
circular sheets of paper
and stack them up, if the
stack is kept vertically
up, we get a Right
circular cylinder, since
it has been kept at right
angles to the base, and
the base is circular.
The area of the rectangular sheet
gives us the curved surface area of
the cylinder. The length of the
sheet is equal to the circumference
of the circular base which is equal
to 2‫ח‬r.
So, curved surface of the cylinder
= area of the rectangular sheet =
length x breadth
= perimeter of the base of the
cylinder x h
= 2 ‫ח‬r x h
Therefore, Curved surface area of a
cylinder = 2‫ח‬rh.
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If the top and the bottom of
the cylinder are also to be
covered, then we need two
circles to do that, each of
radius r, and thus having an
area of ‫ח‬r² each, giving us
the total surface area as
2 ‫ח‬rh + 2 ‫ח‬r² = 2 ‫ח‬r²(r+h) .
So, Total surface area of a
cylinder =2 ‫ח‬r (r +h)
Where h is the height of the
cylinder and r is it’s radius.
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Surface area of a right circular cone
The area of each triangle = ½ x
base of each triangle x l.
So, area of the entire piece of paper
= sum of the areas of all the
triangles
= ½ b1l+ ½b2l+ ½b3l+…
= ½l(b1+b2+b3+…)
= ½ x l x length of entire
curved
boundary of the fig 3
(as b1+b2+b3+… makes up the curved
portion of the fig)
But the curved portion of the figure
makes up the perimeter of the base
of the cone and the circumference
of the base of the cone = 2 ‫ח‬r, where
r is the base radius of the cone.
So, Curved surface area of a cone
= ½ x 1 x 2 ‫ח‬r = ‫ח‬rl.
Where r is it’s base radius and l. it’s
slant height.
TSA of a cone = ‫ח‬r l. + ‫ח‬r² = ‫ח‬r (l+r)
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Surface area of a sphere
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The string which had completely
covered the surface area of the
sphere, has been used to
completely fill the regions of four
circles, all of the same radius as of
the sphere.
This suggests that the surface area
of a sphere of radius r
= 4 times the area of a circle of
radius r =4 x ( ‫ח‬r²)
So,
Surface area of a sphere =4 ‫ח‬r².
Where r is the radius of the sphere.
Evaluation
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A cubic box has each edge 10 cm and another cuboidal box is 12.5 cm
long, 10 cm wide and 8 cm high.
Which box has the grater surface area and by how much?
A cylindrical pillar is 50 cm in diameter and 3.5 m in height. Find the cost
of painting the curved surface of the pillar at the rate of Rs 12.50 per m²
.
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Diameter of the base of a ice cream cone is 10.5 cm and its slant height is
10 cm. Find its curved surface area.
Find the surface area of a sphere of radius: 14 cm.
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Find the radius of a sphere whose surface area is 154 cm
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².