Find (if it exists) the solution for the differential equation: dy/dx = -2xtan(y) given the initial value y(0) = ∏/4 My steps in tackling this, by using the theorem of existence and unicity I would take domain of f(x,y) and the domain of the partial derivative, then figure out the common interval and see whether the initial value is within this interval to find whether a unique solution exists. However, my dilemma. I understand how to find the domain for say dy/dx = √x because its just x in the function, so y would be all real values and x would be ≥ 0. But in my above example there is x AND y in the function, how do I find the domain? Once I figure out whether it has unique solution or not I think I can find solution.