for example i want to make a cylinder of 5 Ltrs. volume. how can i add a formula to keep its volume fixed but i can change its height and radius. i need relativity between its H and r so that if one is varied, the other gets automatically adjusted to maintain the volume to 5L.
Assuming you are using millimetres, one litre = 10^6 cubic millimetres.
So if you want your height to be based on diameter (easier), then your formula needs to be:
h = 5 000 000/(pi*d*d*0.25)
where pi is 3.14159....
h is cylinder height in mm
d is cylinder diameter in mm
I could write the formula for SolidWorks but I can't help you with the exact syntax required for CATIA, sorry.