Answer
F = $\frac{k{m_{1}}{m_{2}}}{r^{2}}$
Work Step by Step
If variables vary directly then y = x, so as one increases the other increases.
If variables vary inversely then y = $\frac{1}{x}$, so as one increases the other decreases.
If variables vary jointly, then the variables are directly proportional, so y = x.
Using the description, F = $\frac{k{m_{1}}{m_{2}}}{r^{2}}$ where k is a constant.
