Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

**CIV****Member**- Registered: 2014-11-09
- Posts: 69

There's a right triangular pyramid with a lateral surface area of 195 root 3 and a slant height of 13. What's the volume?

That's the problem. I got 150 root 3 u^3 as an answer. A friend of mine got 459.86 u^3.

I saw my friends work. I understand some of it, but other parts not so much. Eventually I'll sit down, when I have more time, and figure out what she did.

I just want to get a third person, or more, to try it for an answer. Thanks.

Offline

**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,323

hi CIV,

I'm assuming the three slant faces are equal. I'm getting 300.root(3) as follows:

Area of one slant face = 65.root(3), therefore length of one side of the base = 10. root(3)

By Pythag. height of the triangular base = 15, therefore distance to centre of gravity of the base = 5.

Therefore height of pyramid = 12

So volume = 1/3 base area x height = 1/3 x 1/2 10 root(3) x 15 x 12 = 300 root(3)

Hope that helps,

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

Offline

**CIV****Member**- Registered: 2014-11-09
- Posts: 69

Im going to have to sit down and do it again. You, myself, and my friend all got different answers. Thanks Bob.

Offline

**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,323

hi CIV,

Your answer and mine only differ by a factor of 2, so I think one of us is correct. I've looked at mine several times to see if I've forgotten to half something, but I cannot see an error. That's not to say I'm confident; I often make silly slipups like that.

Please post when you've resolved this so I know where I've gone wrong.

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

Offline