Thread: I need to write this DE

I'm not sure how to write the differential equation that correponds to the following statement:

The rate of change of y with respect to x varies jointly as x and L-y

I'm not sure what is meant by 'varies jointly' in mathematical terms.

Got the following from a website:

"Joint variation means directly, but with two or more variables. An example would be the formula for the area of a triangle with base b and height h: . In words, the area A varies jointly with b and h; the constant of variation is k = 1/2."