Math Is Fun Forum

DessyD

Guest

A pyramid problem

I cant work out these guys

We have a pyramid
n=3
k=√7
b=3
h=2

a=? l=?V=?

And this

Again we have a pyramid
n=3
h=2
l=4
k=√7

a=? b=?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: A pyramid problem

Hi DessyD;

Welcome to the forum!

Do you have a drawing? As LeeC told you on the other forum you have to post a question properly.
I do not understand your variables. For fast answers you have to post lucid questions.

I notice you have posted this problem on 4 forums. It is wasteful to make all those people work on one particular problem.

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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: 10,583

Re: A pyramid problem

hi DessyD

I've put a lettered diagram below.  Please put your measurements in terms of AB = .......  etc.

And similarly, what measurements you want to calculate.

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

DessyD

Guest

Re: A pyramid problem

Guys sorry I didnt mention yesterday Its a pyramidal frustum
actually I did two drawings for these two

n=3
h=2
l=4
k=√7

a=? b=?

Andddd

n=4
h=4√2
l=2√10
a=8

b=? k=?

DessyD

Guest

Re: A pyramid problem

And to say I dont know how it is in english but in both of the tasks the l-s are equal
I dont know the word for that kind of frustum

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: A pyramid problem

hi DessyD

I'm getting closer to helping you, but I am not clear about some of the distances. 

I have made two new diagrams, one for the frustrum of a triangular based pyramid, and one for the square based pyramid.

Please use the letters so say the distances that are given and also to say which ones you want to find.

Be clear about which problem is which.

Bob

---

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

DessyD

Guest

Re: A pyramid problem

Sorry for the late answer guys

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: A pyramid problem

hi DessyD

That's better.  Now I know the lengths.

SQUARE BASED FRUSTRUM:

Draw a line down from C' to meet AC at F.

C'F = EE' = 4√2

Use Pythagoras' theorem to calculate FC.

Use Pythagoras' theorem to calculate AC.  Half it to calculate EC.

Use these answers to calculate EF = E'C'.

Double to get A'C'

Use Pythagoras to calculate A'B'

TRIANGULAR BASED FRUSTRUM will follow.

Additional note.  This one is hard.  It looks like I must do a large number of calculations plus some algebra, to reach a solution.  I want to be sure of each step so it will take a while.

Bob

Last edited by Bob (2010-12-11 04:49:22)

---

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

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: A pyramid problem

hi DessyD

TRIANGULAR BASED FRUSTRUM

n=3
h=2
l=4
k=√7

I am stuck because I don't know which length is 'n'.

Bob

---

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

DessyD

Guest

Re: A pyramid problem

n=3 means its a TRIANGULAR BASED FRUSTRUM
n=4 means square based frustum

DessyD

Guest

Re: A pyramid problem

thanks for helping me for the first one

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: A pyramid problem

hi DessyD

TRIANGULAR BASED FRUSTRUM.

There are three measurements, h, l and k.

But I can work out k from the other two measurements so you have really only given me two bits of information.  This is not enough for me to determine the other lengths in the solid.

I had hoped that ‘n’ was another length; in which case I think it would be easy to calculate all the other lengths. 

Without this, I’m stuck.  Sorry.

Have you completed the square base problem now?

Bob

Last edited by Bob (2010-12-11 23:14:21)

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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