Hi everyone! 1. Is a linear equation the same as a polynomial of first(or 0th) degree? 2. The book 'Mathematics for physicists and engineers' by springer(publisher) states that an example of a linear (DE) equation is 5.dy(x)/dx = x.y(x). Yet I read somewhere else that f(x,y)=a.x.y+b is not linear.(polynomial of 2nd degree). Is one them wrong? Or are both right because in the first example y is a depent variable while in the second example y is an independant variable?