Math Is Fun Forum

cooljackiec

Member

Registered: 2012-12-13

Posts: 186

rectangular prisms

Let  ABCDEFGH be a rectangular prism, as shown. The areas of faces ABCD, ABFE, and ADHE  are 56, 110, and 385, respectively. Find the volume of the rectangular prism.

The total area of all the faces of a rectangular prism is 22, and the total length of all its edges is 24. Find the length of the internal diagonal of the prism.

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I see you have graph paper.
You must be plotting something

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: rectangular prisms

Hi cooljackiec;

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: rectangular prisms

hi cooljackiec,

(i) The tetrahedron.  I think the little one has sides that are half the large one.  So that should fix the ratio of volumes.

LATER EDIT:  I've changed my mind about this one.  See post 10 on

http://www.mathisfunforum.com/viewtopic … 60#p277760

(ii) If you call the sides a, b and c then

ab = 56      ........................(p)
bc = 110     ........................(q)
ca = 385    .........................(r)

If you do equation (p) divide by (q) you'll eliminate b

Then make a the subject and substitute into (r) to solve for c.

Then you can substitute back to get a and b.

(iii) Again call the sides a, b and c.

From the information given

This time you cannot solve for a, b and c because there are only two equations.  But you don't need to, because we only want to get

There's a neat way to get this.  Start with

If you substitute in the values you know you'll be left with the value of a^2 + b^2 + c^2 

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

cooljackiec

Member

Registered: 2012-12-13

Posts: 186

Re: rectangular prisms

hmmm... i used some weird guessing/intuition and got half of the correct answer of number 1...

---

I see you have graph paper.
You must be plotting something

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: rectangular prisms

Hi cooljackiec;

hmmm... i used some weird guessing/intuition and got half of the correct answer of number 1...

That is how I get all my answers. Did you check the answer?

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

cooljackiec

Member

Registered: 2012-12-13

Posts: 186

Re: rectangular prisms

yes, yours was correct

---

I see you have graph paper.
You must be plotting something

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: rectangular prisms

Over at Brilliant - AOPS they say "Work smarter, not harder." But where does that leave guys like me?

Paralysis by analysis.

I say, take thinking out of the solution, it only gets you into trouble. Do not follow Karl Maldens advice, I say leave your brain at home, travel light!

Turn everything into a small template of computational problems and then work on those. The true "Teakettle Principle!"

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.