Question: Triple integral doesn't work

I try to solve triple integraton in Maple with this code.

r := a + (b - a)*z/h;
x1 := sqrt(r^2 - y^2);
V := int(int(int(1, x = -x1 .. x1), y = -r .. r), z = 0 .. h);

but it leaves the last integral dz in the answer and warns: unable to determine if a*h/(-b+a) is between 0 and h; try to use assumptions or use the AllSolutions option
What is the problem?
and i need to get V = Pi*h(a^2 + ab +b^2)/3


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