# Volume computation not returning expected result

I have tried to integrate the expression

(0.5*(3*Cos[t]*Cos[t]-1))*(0.5*(3*Cos[t]*Cos[t]-1))


where t is in [0, π], as a triple integral to get volume. The same function is visualized as shown in the figure below. I know that the surface is enclosed in a box of dimension 0.5 x 0.5 x 2.0, so the box's volume is 0.50 (Unit^3),

However, when I integrate the expression with various methods, I have got,

V = 0.837758 (Unit^3 !)


or

NIntegrate[
(1/2)*(1/2)*(3*Cos[t]*Cos[t] - 1)*(3*Cos[t]*Cos[t] - 1)*r*r*Sin[t],
{r, 0, 1}, {t, 0, Pi}, {p, 0, 2*Pi}]


= 0.837758

My point is: It can be clearly seen that the surface is well encapsulated in the box, so its volume is expected < 0.50 Unit^3, but I'm not getting that result.

So I am confused here. Can anyone help me to figure what went wrong?

(1/2)*(1/2)*(3*Cos[t]*Cos[t] - 1)*(3*Cos[t]*Cos[t] - 1) is not the density, it is the maximal radius of the body. So you evaluate the wrong integral. Try this:

NIntegrate[
r r Sin[t],
{t, 0, Pi},
{p, 0, 2*Pi},
{r, 0, 1/4 (3 Cos[t] Cos[t] - 1)^2}
]


0.221784

• Many Thanks it agrees with stochastic int. methods..... – numeric Nov 28 '18 at 6:46
• You're welcome. – Henrik Schumacher Nov 28 '18 at 6:54