a) The independent variable is r, the radius. You can use any allowable number for r. The dependent variable is V, the volume. Once you choose a value for r, the value of V will depend on that value of r.
b) The domain is all real numbers greater than 0. A radius cannot be zero or a negative number.
c) The output of V(8) is the function V(r) evaluated at r = 8, so it is the volume of a sphere of radius 8 units.
This gives the two sides of the equation as 30 = x
Therefore, x = 30
This is a simple equation. It can easily calculate the value of x given the value on the other side of the equation. Therefore, x is a dependent variable in this system of equation. And just like every system of equation, the simple equation problem requires that the two sides of the equation are always kept in balance or equilibrium.