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# FInd the Unknown Dimension in the Polyhedron

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The edge length of a cube with diagnal of 9 ft. What is the edge length?

liveevillevi  Apr 19, 2017
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#1
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Let's just look at one face of this cube.

(Since it is a cube, all the faces are the exact same.)

The edge length is labeled "s".

Since the diagonal forms the hypotenuse of a right triangle,

and both legs are "s",

we can use the Pythagorean theorem to find s.

s2 + s2 = 92

2s2 = 81

s2 = 40.5

s = $$+\sqrt{40.5} \approx 6.364 \text{ feet}$$

hectictar  Apr 19, 2017
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where did u get the diagram dude? Thats awesome!

liveevillevi  Apr 19, 2017
#3
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Thanks! I just made it using paint

hectictar  Apr 19, 2017
#4
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Woudld that be a website?

liveevillevi  Apr 19, 2017
#5
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No it is not a website haha... it is a program that I think comes standard on Microsoft computers..it is at least on computers I've seen.

It's called " MS Paint "

hectictar  Apr 19, 2017
#6
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Hectictar has given the edge length if the diagonal is across one face of the cube ...DB as shown below.......however.......if the diagonal is considered to be AB....we have...

AB  =  √[3s^2]

9  = √[3s^2]      square both sides

81  =  3 s^2      divide both sides by 3

27  =  s^2         take the square root of both sides

3√3  ft   =  s  ≈  5.19  ft.

CPhill  Apr 19, 2017
#7
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Well, thanks, but yeah, I did need the 3d answer with x,y,z distance :)

liveevillevi  Apr 20, 2017

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