IMPORTANT
FORMULAE
I. CUBOID
Let length = 1,
breadth = b and height = h units. Then, 1. Volume = (1 x b x h) cubic units.
2. Surface area= 2(lb + bh + lh) sq.units.
3. Diagonal.=Öl2 +b2
+h2 units
II.
CUBE
Let each edge of
a cube be of length a. Then,
1. Volume = a3 cubic units.
2. Surface area = 6a2 sq. units.
3. Diagonal = Ö3 a units.
III. CYLINDER
Let radius of
base = r and Height (or length) = h. Then,
1. Volume = (P r2h) cubic units.
2. Curved surface area = (2P rh). units.
3. Total surface area =2Pr (h+r) sq. units
IV.
CONE
Let radius of
base = r and Height = h. Then,
1. Slant height, l =Ö h2+r2
2.
Volume =
(1/3) Pr2h cubic units.
3. Curved
surface area
= (Prl) sq. units.
4. Total surface area = (Prl + Pr2 ) sq. units.
V. SPHERE
Let the radius
of the sphere be r. Then,
1. Volume = (4/3)Pr3 cubic units.
2. Surface area = (4Pr2) sq. units.
VI.
HEMISPHERE
Let the radius
of a hemisphere be r. Then,
1. Volume = (2/3)Pr3 cubic units.
2. Curved
surface area
= (2Pr2) sq. units.
3. Total surface area = (3Pr2) units.
Remember: 1 litre = 1000 cm3.
SOLVED EXAMPLES
.
Ex. 1. Find the volume and surface area of a cuboid
16 m long, 14 m broad and
7 m high.
Sol. Volume = (16 x
14 x 7) m3 = 1568 m3.
1
Surface area = [2 (16 x 14 + 14 x 7 + 16 x 7)] cm2 = (2 x
434) cm2 = 868 cm2.
Ex. 2. Find the length of the longest pole that can be placed in a room
12 m long
8m broad and 9m high.
i
Sol. Length of longest pole =
Length of the diagonal of the room
=
Ö(122+82+92=
.Ö(289)= 17 m.
Ex. 3. Tbe volume of a wall, 5 times as high as it
is broad and 8 times as long as
it is high, is 12.8 cu.
metres. Find the breadth of the wall.
Sol. Let the breadth of the wall be
x metres.
Then, Height = 5x metres and
Length = 40x metres.
\x * 5x * 40x = 12.8 Û x3=12.8/200 = 128/2000 = 64/1000
So, x = (4/10) m =((4/10)*100)cm = 40 cm
Ex. 4. Find the number of bricks, each measuring 24 cm x 12 cm x 8 cm,
required to construct a wall 24 m long, 8m high and 60 cm thick, if 10% of the
wall is filled with mortar?
Sol. Volume of the wall = (2400
x 800 x 60) cu. cm.
Volume of bricks = 90% of the
volume of the wall
=((90/100)*2400
*800 * 60)cu.cm.
Volume of 1 brick = (24 x 12 x 8) cu.
cm.
\Number of
bricks=(90/100)*(2400*800*60)/(24*12*8)=45000.
Ex.
5. Water flows into a tank 200 m x 160 m througb a rectangular pipe of
1.5m x 1.25 m @ 20 kmph . In what time (in minutes)
will the water rise by 2
metres?
Sol. Volume required in the tank = (200 x 150 x 2) m3
= 60000 m3.
.
.
Length of water column
flown in1 min =(20*1000)/60 m =1000/3 m
Volume flown per minute = 1.5 * 1.25 *
(1000/3) m3 = 625 m3.
\ Required time = (60000/625)min = 96min
Ex. 6. Tbe dimensions of an
open box are 50 cm, 40 cm and 23 cm. Its thickness is
2 cm. If 1 cubic cm of metal
used in the box weighs 0.5 gms, find the weight of the box.
Sol.
Volume of the metal used in the box = External Volume - Internal Volume
=
[(50 * 40 * 23) - (44 * 34 * 20)]cm3
=
16080 cm3
\ Weight of the metal
=((16080*0.5)/1000) kg = 8.04 kg.
Ex. 7. The diagonal of a
cube is 6Ö3cm. Find its volume and
surface area.
Sol. Let the edge of the cube be a.
\Ö3a = 6../3 _ a = 6.
So,Volume = a3 = (6 x 6 x 6) cm3
= 216 cm3.
Surface area = 6a2 = (6 x
6 x 6) cm2 == 216 cm2.
Ex. 8. The surface area of a
cube is 1734 sq. cm. Find its volume.
Sol. Let the edge of the cube
bea. Then,
6a2 = 1734 Þ a2 = 289 => a = 17 cm.
\ Volume = a3 =
(17)3 cm3 = 4913 cm3.
Ex. 9. A rectangular block 6 cm by 12 cm by 15 cm is cut
up into an exact number of equal cubes. Find the least possible number of
cubes.
Sol. Volume of the block = (6 x
12 x 15) cm3 = 1080 cm3.
Side of the largest cube = H.C.F. of 6 cm, 12
cm, 15 cm = 3 cm.
Volume of this cube = (3 x 3 x 3) cm3
= 27 cm3.
Number of cubes = 1080/27
= 40.
Ex.l0. A cube of edge 15 cm is immersed completely
in a rectangular vessel containing water
. If the dimensions of the base of vessel are 20 cm x 15 cm, find the rise in
water level.
Sol. Increase in volume = Volume
of the cube = (15 x 15 x 15) cm3.
\Rise in water level = volume/area = (15 x 15
x 15)/(20 x 15) cm = 11.25 cm.
Ex.
11. Three solid cubes of sides 1 cm, 6 cm and 8 cm are melted to form a new
cube. Find the surface area
of the cube so formed.
Sol.
Volume of
new cube = (13 + 63 + 83) cm+ = 729 cm3.
Edge of new cube = 3Ö729 cm = 9 cm.
\ Surface area of the new cube = (6 x 9 x 9) cm2
= 486 cm2.
Ex. 12. If each edge of a cube is increased
by 50%, find the percentage increase in
Its surface area. "
Sol. Let original length of each
edge = a.
Then,
original surface area = 6a2.
New
edge = (150% of a) = (150a/100) = 3a/2
New
surface area = 6x (3a/2)2 =
27a2/2
Increase percent in surface area = ((15a2)
x ( 1 ) x 100)% = 125%
2
6a2
Ex. 13. Two cubes have their volumes in the ratio 1 : 27. Find the
ratio of their
surface areas.
Sol.
Let their edges be a and b. Then,
a3/b3
= 1/27 (or) (a/b)3 = (1/3)3 (or) (a/b) = (1/3).
\Ratio of their surface area
= 6a2/6b2 = a2/b2 = (a/b)2
= 1/9, i.e. 1:9.
Ex.14.Find
the volume , curved surface area and the total surface area of a cylinder with
diameter of base 7 cm and height 40 cm.
Sol. Volume = ∏r2 h =
((22/7)x(7/2)x(7/2)x40) = 1540 cm3. .
Curved surface area = 2∏rh =
(2x(22/7)x(7/2)x40)= 880 cm2 .
Total surface area = 2∏rh + 2∏r2
= 2∏r (h + r)
= (2 x
(22/7) x (7/2) x (40+3.5)) cm2
= 957 cm2
Ex.15. If the capacity of a cylindrical tank is 1848
m3 and the diameter of its base
is 14 m, then find the depth
of the tank.
Sol. Let the depth of the tank be h metres. Then,
∏ x 72 x h =
1848 Þ h =
(1848 x (7/22) x (1/49) = 12 m
Ex.16. 2.2 cubic dm of lead is to be drawn into a
cylindrical wire 0.50 cm
diameter. Find the length of
the wire in metres.
Sol. Let the length of the wire be h metres. Then,
∏ (0.50/(2 x 100))2 x h = 2.2/1000
Þ h = ( (2.2/1000) x (100 x
100)/(0.25 x 0.25) x (7/22) ) = 112 m.
Ex.
17. How many iron rods, each of length 7 m and diameter 2 cm can be made
out of 0.88 cubic metre of iron?
Sol. Volume of 1 rod = (( 22/7)
x (1/100) x (1/100) x 7 ) cu.m = 11/5000 cu.m
Volume
of iron = 0.88 cu. m.
Number
of rods = (0.88 x 5000/11) = 400.
Ex. 18. The radii of two cylinders are in the ratio
3: 5 and their heights are in tbe
ratio of 2 : 3. Find the
ratio of their curved surface areas.
Sol.
Let the radii of the cylinders be 3x, 5x and their heights be 2y, 3y
respectively. Then
Ratio of their curved surface area = 2∏
X 3x X 2y = 2/5 = 2.5
2∏ X 5x X 3y
Ex. 19. If 1 cubic cm of cast iron weighs 21 gms, then find the eight
of a cast iron
pipe of length 1 metre with a
bore of 3 cm and in which thickness of the metal is 1 em.
Sol. Inner radius = (3/2) cm =
1.5 cm, Outer radius = (1.5 + 1) = 2.5 cm.
\Volume of iron = [∏ x (2.5)2
x 100 - ∏ x (1.5)2 x 100] cm3
= (22/7) x
100 x [(2.5)2 - (1.5)2] cm3
= (8800/7)
cm3
Weight of the
pipe = ((8800/7) x (21/1000))kg = 26.4 kg.
Ex. 20. Find the slant height, volume, curved
surface area and the whole surface
area
of a cone of radius 21 cm and height 28 cm.
Sol. Here, r = 21 cm and h = 28
cm.
\ Slant height, l = Ör2 + h2 = Ö (21)2 + (28)2 = Ö 1225 = 35cm
Ex. 21. Find the length of canvas 1.25 m wide required to build a conical tent of base radius 7 metres and height 24 metres.
Sol. Here, r = 7m and h = 24 m.
So,l = Ö(r2 + h2) = Ö(72 + 242) = Ö (625) = 25 m.
Area of canvas = Õrl=((22/7)*7*25)m2 = 550 m2 .
Length
of canvas = (Area/Width) = (550/1.25) m = 440 m.
Ex. 22. The heights of two right circular cones are in the ratio 1 : 2 and the
perimeters of their bases
are in the ratio 3 : 4. Find the ratio of their volumes.
Sol. Let the radii of their
bases be r and R and their heights be h and 2h respectively.
Then,(2Õr/2ÕR)=(3/4) Þ R=(4/3)r.
\ Ratio of volumes = (((1/3)Õ r2h)/((1/3)Õ(4/3r)2(2h)))=9 : 32.
Ex. 23. The radii of the bases of a cylinder and a cone
are in the ratio of 3 : 4 and It heights are in the ratio 2 : 3. Find the ratio
of their volumes.
Sol. Let the radii of the
cylinder and the cone be 3r and 4r and their heights be 2h and
3h respectively.
:. Volume of
cylinder = Õ x (3r)2 * 2h = 9/8 = 9 : 8.
Volume of cone (1/3)Õr2 * 3h
Ex. 24. A conical vessel, whose internal radius is
12 cm and height 50 cm, is full of
liquid. The contents are emptied into a cylindrical vessel with internal radius
10 cm. Find the height to which the liquid rises in the cylindrical vessel.
Sol.
Volume of the liquid in the cylindrical vessel
=
Volume of the conical vessel
= ((1/3)*
(22/7)* 12 * 12 * 50) )cm3 = (22 *4 *12 * 50)/7 cm3.
Let the height of the liquid
in the vessel be h.
Then (22/7)*10*10*h =(22*4*12*50)/7 or h = (4*12*50)/100 = 24 cm
Ex.
25. Find the volume and surface area of a sphere of radius 10.5 cm.
3
3
Sol. Volume = (4/3)Õr3 =(4/3)*(22/7)*(21/2)*(21/2)*(21/2)
cm3 = 4851 cm3.
Surface area = 4Õr 2 =(4*(22/7)*(21/2)*(21/2)) cm2
= 1386 cm2
Ex. 26. If the radius of a sphere is increased by
50%, find the increase percent in
volume and the increase
percent in the surface area.
Sol.
Let original radius = R. Then, new radius = (150/100)R=(3R/2)
Original volume = (4/3)ÕR3, New volume =
(4/3)Õ(3R/2)3 =(9ÕR3/2)
Increase % in volume=((19/6)ÕR3)*(3/4ÕR3)*100))% = 237.5%
Original surface area =4ÕR2. New surface
area = 4Õ(3R/2)2=9ÕR2
Increase % in surface area =(5ÕR2/4ÕR2) * 100) % = 125%.
Ex. 27. Find the number of lead balls, each 1 cm in diameter that can be
a sphere of diameter 12 cm.
Sol. Volume of larger sphere =
(4/3)Õ*6*6*6) cm3 = 288Õ cm3.
Volume of 1 small lead ball =
((4/3)Õ*(1/2)*(1/2)*(1/2)) cm3
= Õ/6 cm3.
\ Number of lead balls = (288Õ*(6/Õ)) = 1728.
Ex.28.How
many spherical bullets can be made out of a lead cylinder 28cm high and with
radius 6 cm, each bullet being 1.5 cm in diameter ?
Sol. Volume of cylinder = (∏ x 6
x 6 x 28 ) cm3 = ( 9∏/16) cm3.
Number of bullet = Volume of cylinder = [(36 x 28)∏ x 16] /9∏ = 1792.
Volume of each bullet
Ex.29.A
copper sphere of diameter 18cm is drawn into a wire of diameter 4 mm Find the
length of the wire.
Sol.
Volume of
sphere = ((4∏/3) x 9 x 9 x 9 ) cm3
= 972∏ cm3
Volume of sphere = (∏ x 0.2 x 0.2 x h ) cm3
\ 972∏ = ∏ x (2/10) x (2/10)
x h Þ h = (972 x 5 x 5 )cm = [( 972 x 5 x5 )/100
] m
= 243m
Ex.30.Two
metallic right circular cones having their heights 4.1 cm and 4.3 cm and the
radii of their bases 2.1 cm each, have been melted together and recast into a
sphere. Find the diameter of the sphere.
Sol. Volume of sphere = Volume
of 2 cones
= ( 1 ∏ x (2.102) x
4.1 + 1 ∏ x (2.1)2 x 4.3)
3
3
Let the radius of sphere be R
\(4/3)∏R3 =
(1/3)∏(2.1)3 or R = 2.1cm
Hence , diameter of the sphere = 4.2.cm
Ex.31.A Cone and a sphere
have equal radii and equal volumes. Find the ratio of the sphere of the
diameter of the sphere to the height of the cone.
Sol. Let radius of each be R
and height of the cone be H.
Then, (4/3) ∏ R3 = (1/3) ∏ R2H
(or) R/H = ¼ (or) 2R/H = 2/4 =1/2
\Required ratio = 1:2.
Ex.32.Find
the volume , curved surface area and the total surface area of a hemisphere of
radius 10.5 cm.
Sol. Volume = (2∏r3/3)
= ((2/3) x (22/7) x (21/2) x (21/2) x (21/2))cm3
= 2425.5 cm3
Curved surface area = 2∏r3 =
(2 x (22/7) x (21/2) x (21/2))cm2
=693 cm2
Total surface area = 3∏r3 =
(3 x (22/7) x (21/2) x (21/2))cm2
= 1039.5 cm2.
Ex.33.Hemispherical bowl of
internal radius 9 cm contains a liquid. This liquid is to be filled into
cylindrical shaped small bottles of diameter 3 cm and height 4 cm. How many
bottles will be needed to empty the bowl ?
Sol. Volume of bowl = ((2∏/3)
x 9 x 9 x 9 ) cm3 = 486∏ cm3.
Volume of 1 bottle = (∏ x
(3/2) x (3/2) x 4 ) cm3 = 9∏ cm3
Number of bottles =
(486∏/9∏) = 54.
Ex34.A Cone,a
hemisphere and a cylinder stand on equal bases and have the same height.Find
ratio of their volumes.
Sol. Let R be the radius of each
Height of the hemisphere = Its radius = R.
\Height of each = R.
Ratio of volumes = (1/3)∏ R2 x
R : (2/3)∏R3 : ∏ R2 x R = 1:2:3
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