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Finding volume using equilateral triangles?

Posted 1 year ago

I've got a solid I'm trying to find the volume of. I've gotten down the process in Mathematica as to how to do this if it's being rotated around an axis or even a line, but let's assume we have a cross section that is between two curves and bounded by x > 0.

We're going to find the volume of the solid if the region is cross sections of equilateral triangles perpendicular to the x axis here. I've already got code for the two endpoints...is it possible to have Mathematica do something like this?

This is what I have right now...but I'm not sure if this is being done properly for the situation.

f[x_] := 3x^3 - 0.75x^5
g[x_] := x^2 - 6x + 2
{x1, x2}=x/. Solve[f[x]==g[x] && x > 0,x, Reals];
sideLength=Sqrt[3](f[x]-g[x])/2;
volumeTri=Integrate[1/4*Pi*sideLength^2, {x, x1, x2}]

I'm getting a result here but I'm not sure if this is how it's done in Mathematica, nor could I find any supporting documentation. I don't need any visual output, just the volume of the solid in which the region between those curves (for x > 0) has cross sections perpendicular to the x-axis that are equilateral triangles.

Is this how it's done in Mathematica or am I missing something? Any suggestions on efficiency to improve the code are also welcome.

POSTED BY: Scott B
4 Replies

5-4-2023
How is it bounded by x>0 if that is open?
It also is not an interval.
Could that be the problem?
Apologies if I'm off.

Posted 1 year ago

Ahhh I think I misworded that...basically I'm looking at the curves formed by the points of intersection where x > 0. They do intersect where x < 0.

Anyhow I think I did solve this issue...I simply did this by creating a line to find the area:

area = (Sqrt[3]/4) * sideLength^2;

and integrating that, while simplifying sideLength to be just the difference of the two functions.

Basically, writing another line of code to simplify things and making it so only "area" was integrated.

It seemed to work. I'm surprised though that there doesn't seem to be an easier command in Mathematica for anything like this--maybe I'm wrong?

POSTED BY: Scott B

Crossposted here.

POSTED BY: Rohit Namjoshi
Posted 1 year ago

Yes, because I can't get an answer there I figured I"d try there too. I'm just really frustrated because no matter how much I research this I can't get an answer one way or another if Mathematica can actually do this!

POSTED BY: Scott B
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