how we can knew the minimum value of the point on a curve?
that depends on the tools you have at your disposal. Some curves have special properties, like the vertex of the parabola
ax^2+bx+c is at x = -b/2a, and so forth.
If you have access to calculus, you can find minima on a wider variety of curves.
The most general tool would be a graphing calculator, where you can just see the minima.