Write a general formula to describe each variation. Let k be the constant of proportionality for each variation.

1. T varies jointly with the cube root of x and the square of d.

T = cuberoot{x}(d^2)k

2. z varies directly with the sum of the cube of x and the square of y.

z = (x^3 + y^2)k

3. The cube of z varies directly with the sum of the squares of x and y.

z^3 = (x^2 + y^2)k

Is any of this correct?