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Off-Topic Discussion => General Discussion => Topic started by: kasperl on December 19, 2004, 07:48:55 am

Title: basic math help
Post by: kasperl on December 19, 2004, 07:48:55 am

OK, I'm studying for an exam and me and the answer-booklet don't agree. Please help me.

There is a piramid, PQRS T with PQ=QR=RS=PS=6 and height=8.
There is a cube thingy in this piramid, ABCDEFGH. A B C and D are in the middelpoints of PT, QT, RT and ST. A B C and D are in the groundsquare of the piramid.

Calculate the volume inside the piramide but outside the cube.

I get the following:
Volume(piramid)=1/3*6*6*8=96
Volume(cuboid)=3*3*4=36
Volume(piramid-cuboid)=96-36=60.

The answer, according to the booklet, is 36. What's wrong here?

Title: basic math help
Post by: Setekh on December 19, 2004, 08:33:16 am

The volume of the pyramid is right. Either your solution for the cube is the problem, or the answer is indeed wrong. Knowing that, I'm still unsure about the cube thingy. Need some more information about EFGH... because if I've drawn this diagram correctly, if ABCD is the base of the cube, then most of the cube is not actually inside the pyramid. :nervous:

Title: basic math help
Post by: kasperl on December 19, 2004, 08:38:02 am

ABCD is the base of the cube. You create ABCD by drawing a line straight down from the middelpoints of the ribs of the piramid.

EDIT: Here are the answers, mind, it's a big PDF. It's question 1b.

http://www.getalenruimte.epn.nl/getalenruimte_editie1998/ContentLeerling/vwoNGNT3/1/x_Hele_hoofdstuk/2_Uitwerking_algemene_herhaling/1_antwoorden.pdf

The diagrams of 1a are of the thing we're talking about.

Title: basic math help
Post by: Setekh on December 19, 2004, 08:45:10 am

But is T the apex of the pyramid? In that case, that would make A,B,C,D the midpoints of PT, QT, RT, ST (the "ribs" of the pyramid) - is that more accurate than your description above?

Title: basic math help
Post by: kasperl on December 19, 2004, 08:45:39 am

Oh, I just can't friggin type, can I?

You're right. I should learn how to type.

Title: basic math help
Post by: Setekh on December 19, 2004, 08:52:00 am

That's okay. :)

Checking out their answer, they're definitely wrong. They've listed [(1/3)*6*6*8 - 3*3*4] as equal to [72-36]... when rather, it's [96-36], as you spotted earlier. You're correct.

Title: basic math help
Post by: kasperl on December 19, 2004, 08:53:19 am

Thank you.


Now, I'm going to load up a truck full with fire-arms and off to that publisher. I'll be right back.