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# Math : 3-D Geometries

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01 Oct 2010, 17:18
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3-D Geometries

This post is a part of [GMAT MATH BOOK]

created by: shrouded1

Scope

The GMAT often tests on the knowledge of the geometries of 3-D objects such cylinders, cones, cubes & spheres. The purpose of this document is to summarize some of the important ideas and formulae and act as a useful cheat sheet for such questions

Cube

A cube is the 3-D generalisation of a square, and is characterized by the length of the side, $$a$$. Important results include :

• Volume = $$a^3$$
• Surface Area = $$6a^2$$
• Diagnol Length = $$\sqrt{3}a$$

Cuboid

A cube is the 3-D generalisation of a rectangle, and is characterized by the length of its sides, $$a,b,c$$. Important results include :

• Volume = $$abc$$
• Surface Area = $$2(ab+bc+ca)$$
• Diagnol Length = $$\sqrt{a^2+b^2+c^2}$$

Cylinder

A cylinder is a 3-D object formed by rotating a rectangular sheet along one of its sides. It is characterized by the radius of the base, $$r$$, and the height, $$h$$. Important results include :

• Volume = $$\pi r^2 h$$
• Outer surface area w/o bases = $$2 \pi r h$$
• Outer surface area including bases = $$2 \pi r (r+h)$$

Cone

A cone is a 3-D object obtained by rotating a right angled triangle around one of its sides. It is charcterized by the radius of its base, $$r$$, and the height, $$h$$. The hypotenuse of the triangle formed by the height and the radius (running along the diagnol side of the cone), is known as it lateral height, $$l=\sqrt{r^2+h^2}$$. Important results include :

• Volume = $$\frac{1}{3} \pi r^2 h$$
• Outer surface area w/o base = $$\pi r l =\pi r \sqrt{r^2+h^2}$$
• Outer surface area including base = $$\pi r (r+l)=\pi r (r+\sqrt{r^2+h^2})$$

Sphere

A sphere is a 3-D generalisation of a circle. It is characterised by its radius, $$r$$. Important results include :

• Volume = $$\frac{4}{3} \pi r^3$$
• Surface Area= $$4 \pi r^2$$

A hemisphere is a sphere cut in half and is also characterised by its radius $$r$$. Important results include :

• Volume = $$\frac{2}{3} \pi r^3$$
• Surface Area w/o base = $$2 \pi r^2$$
• Surface Area with base = $$3 \pi r^2$$

Some simple configurations

These may appear in various forms on the GMAT, and are good practice to derive on one's own :

1. Sphere inscribed in cube of side $$a$$ : Radius of sphere is $$\frac{a}{2}$$
2. Cube inscribed in sphere of radius $$r$$ : Side of cube is $$\frac{2r}{\sqrt{3}}$$
3. Cylinder inscribed in cube of side $$a$$ : Radius of cylinder is $$\frac{a}{2}$$; Height $$a$$
4. Cone inscribed in cube of side $$a$$ : Radius of cone is $$\frac{a}{2}$$; Height $$a$$
5. Cylinder of radius $$r$$ in sphere of radius $$R$$ ($$R>r$$) : Height of cylinder is $$2\sqrt{R^2-r^2}$$

Examples

Example 1 : A certain right circular cylinder has a radius of 5 inches. There is oil filled in this cylinder to the height of 9 inches. If the oil is poured completely into a second right cylinder, then it will fill the second cylinder to a height of 4 inches. What is the radius of the second cylinder, in inches?

A. 6
B. 6.5
C. 7
D. 7.5
E. 8

Solution : The volume of the liquid is constant.
Initial volume = $$\pi * 5^2 * 9$$
New volume = $$\pi * r^2 * 4$$
$$\pi * 5^2 * 9 = \pi * r^2 * 4$$
$$r = (5*3)/2 = 7.5$$

Example 2 : A spherical balloon has a volume of 972 $$\pi$$cubic cm, what is the surface area of the balloon in sq cm?

A) 324
B) 729
C) 243 $$\pi$$
D) 324 $$\pi$$
E) 729 $$\pi$$

Solution : $$V=\frac{4}{3} \pi r^3$$
$$r = (\frac{3V}{4\pi})^{\frac{1}{3}} = (\frac{3 * 972 * \pi}{4 * \pi})^{\frac{1}{3}} = (3*243)^{1/3} = (3^6)^{1/3} = 9$$
$$A=4 \pi r^2 = 4 * \pi * 9^2=324 \pi$$

Example 3 : A cube of side 5cm is painted on all its side. If it is sliced into 1 cubic centimer cubes, how many 1 cubic centimeter cubes will have exactly one of their sides painted?

A. 9
B. 61
C. 98
D. 54
E. 64

Solution : Notice that the new cubes will be each of side 1Cm. So on any face of the old cube there will be 5x5=25 of the smaller cubes. Of these, any smaller cube on the edge of the face will have 2 faces painted (one for every face shared with the bigger cube). The number of cubes that have exacly one face painted are all except the ones on the edges. Number on the edges are 16, so 9 per face.

There are 6 faces, hence 6*9=54 smaller cubes with just one face painted.

Example 4 : What is the surface area of the cuboid C ?
(1) The length of the diagnol of C is 5
(2) The sum of the sides of C is 10

Solution : Let the sides of cuboid C be $$x,y,z$$
We know that the surface area is given be $$2(xy+yz+zx)$$
(1) : Diagnol = $$\sqrt{x^2+y^2+z^2}=5$$. Not sufficient to know the area
(2) : Sum of sides = $$x+y+z=10$$. Not sufficient to know the area
(1+2) : Note the identity $$(x+y+z)^2=x^2+y^2+z^2+2(xy+yz+zx)$$
Now we clearly have enough information.
$$2(xy+yz+zx) = 10^2 - 5^2 = 75$$
Sufficient

3-D Geometry Questions:

3-d-geometry-questions-171024.html

Some Other 3-D Problems

Sphere & Cube
Sphere & Cylinder
Cylinder & Cuboid
Cylinder & Cuboid II
Cylinder
Cube
Cube II
Cone
Cube III
Cylinder
Hemisphere
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Last edited by Bunuel on 28 May 2014, 03:17, edited 7 times in total.
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Re: Math : 3-D Geometries [#permalink]

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01 Oct 2010, 18:51
1
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Thanks a ton for posting this +1 Kudo
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02 Oct 2010, 08:16
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Added some solved examples to the post
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02 Oct 2010, 09:32
Wow.. amazing stuff. +1
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Re: Math : 3-D Geometries [#permalink]

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02 Oct 2010, 22:39
1
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Expert's post
WOOOOOOOOOOOOOOOOOOOOOOOOOOOOOWWWWWWWWWWWWWWWWWWWW

I sense Bunuel has competition!
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Re: Math : 3-D Geometries [#permalink]

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02 Oct 2010, 23:09
Mathematica be the shizzles.
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08 Oct 2010, 04:38
Thank you very much!
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Re: Math : 3-D Geometries [#permalink]

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09 Oct 2010, 06:01
Kudos! for writing that up. 3-D is what was missing in GMAT Math Book, gotta update it in compiled GMAT Math Book pdf.
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03 Jun 2011, 09:17
Thnks for sharing....
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Re: Math : 3-D Geometries [#permalink]

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25 Aug 2012, 00:34
Thanks a tons for this wonderful post.

This question/ configuration is quite popular(already explained by you in other threads), you might include it as well.
Inscribed Sphere touching the edges of cube.
Cube edge length=a; Radius of sphere=a/2; diagnoal of cube =(3^1/2)*a
The shortest length from edge of the cube to sphere's surface is given by
half the diagnol of cubeminus the raduis of sphere.

[(3^1/2)*a]/2-a/2 =a/2*[(3^1/2)-1]
Attachments

sphere.png [ 19.43 KiB | Viewed 21935 times ]

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Re: Math : 3-D Geometries [#permalink]

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10 Jul 2013, 23:07
Bumping for review*.

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Re: Math : 3-D Geometries [#permalink]

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01 Jul 2014, 08:13
m]r = (\frac{3V}{4\pi})^{\frac{1}{3}} = (\frac{3 * 972 * \pi}{4 * \pi})^{\frac{1}{3}} = (3*243)^{1/3} = (3^6)^{1/3} = 9[/m]

How exactly did you get from (\frac{3 * 972 * \pi}{4 * \pi})^{\frac{1}{3}} to this ---> = (3*243)^{1/3}

Do we have to multiply the 3 and 972 inside the parntheses? and what happens to the 4?

Thanks
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Re: Math : 3-D Geometries [#permalink]

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01 Jul 2014, 08:15
sagnik2422 wrote:
m]r = (\frac{3V}{4\pi})^{\frac{1}{3}} = (\frac{3 * 972 * \pi}{4 * \pi})^{\frac{1}{3}} = (3*243)^{1/3} = (3^6)^{1/3} = 9[/m]

How exactly did you get from (\frac{3 * 972 * \pi}{4 * \pi})^{\frac{1}{3}} to this ---> = (3*243)^{1/3}

Do we have to multiply the 3 and 972 inside the parntheses? and what happens to the 4?

Thanks

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Re: Math : 3-D Geometries [#permalink]

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01 Jul 2014, 08:27
A cube of side 5cm is painted on all its side. If it is sliced into 1 cubic centimer cubes, how many 1 cubic centimeter cubes will have exactly one of their sides painted?

A. 9
B. 61
C. 98
D. 54
E. 64

Solution : Notice that the new cubes will be each of side 1Cm. So on any face of the old cube there will be 5x5=25 of the smaller cubes. Of these, any smaller cube on the edge of the face will have 2 faces painted (one for every face shared with the bigger cube). The number of cubes that have exacly one face painted are all except the ones on the edges. Number on the edges are 16, so 9 per face.

There are 6 faces, hence 6*9=54 smaller cubes with just one face painted.

QUESTION : Number on the edges are 16, so 9 per face.

HOW DO WE KNOW THERE ARE 16 EDGES ? AND FROM THIS HOW IS 9 CALCULATED ?

THANKS
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Re: Math : 3-D Geometries [#permalink]

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01 Jul 2014, 09:24
sagnik2422 wrote:
A cube of side 5cm is painted on all its side. If it is sliced into 1 cubic centimer cubes, how many 1 cubic centimeter cubes will have exactly one of their sides painted?

A. 9
B. 61
C. 98
D. 54
E. 64

Solution : Notice that the new cubes will be each of side 1Cm. So on any face of the old cube there will be 5x5=25 of the smaller cubes. Of these, any smaller cube on the edge of the face will have 2 faces painted (one for every face shared with the bigger cube). The number of cubes that have exacly one face painted are all except the ones on the edges. Number on the edges are 16, so 9 per face.

There are 6 faces, hence 6*9=54 smaller cubes with just one face painted.

QUESTION : Number on the edges are 16, so 9 per face.

HOW DO WE KNOW THERE ARE 16 EDGES ? AND FROM THIS HOW IS 9 CALCULATED ?

THANKS

This solution should be edited.

A cube has 12 edges, 6 faces and 8 vertices:
Attachment:

faces-edges-vertices.png [ 13.61 KiB | Viewed 14300 times ]

As for the question. Look at the image below:
Attachment:

MagicCube5x5.jpg [ 71.65 KiB | Viewed 16967 times ]
Little cubes with exactly one painted side will be those 3*3=9, which are in the center of each face. (6 faces)*(9 per each) = 54.

Similar questions to practice:
the-entire-exterior-of-a-large-wooden-cube-is-painted-red-155955.html
a-big-cube-is-formed-by-rearranging-the-160-coloured-and-99424.html
64-small-identical-cubes-are-used-to-form-a-large-cube-151009.html
a-wooden-cube-whose-edge-length-is-10-inches-is-composed-of-162570.html
if-a-4-cm-cube-is-cut-into-1-cm-cubes-then-what-is-the-107843.html
a-large-cube-consists-of-125-identical-small-cubes-how-110256.html

3-D Geometry Questions to practice: 3-d-geometry-questions-171024.html
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Re: Math : 3-D Geometries [#permalink]

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19 Feb 2015, 12:28
Can you please explain me how i resolve this equation? I cant seem to do it right.
its from the first example:

"Solution : The volume of the liquid is constant.
Initial volume = \pi * 5^2 * 9
New volume = \pi * r^2 * 4
\pi * 5^2 * 9 = \pi * r^2 * 4
r = (5*3)/2 = 7.5"
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Re: Math : 3-D Geometries [#permalink]

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19 Feb 2015, 18:56
Hi bogdanbb,

Your approach and solution are correct (the radius is 7.5). What part about it do you not understand?

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01 May 2016, 06:31
Hi bb,

None of the images are visible. Kindly check the image links. Thanks
Math : 3-D Geometries   [#permalink] 01 May 2016, 06:31
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