cuberoot(r ** 2)

https://www.wolframalpha.com/input?i=cuberoot%28r+**+2%29

R = r ** 2= rx ** 2 + ry**2. to get r, you need first sqrt(r) which is R ** 0.5.

so, r**3 = R ** (0.5*3) = R ** 1.5

Hope this explains your question.