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 P: 369 Hi there, I am reading an introduction on solving ODE, there the exact equation is mentioned. Suppose a ODE is in the following form $$M(x, y) + N(x, y)\frac{dy}{dx} = 0$$ assume there is a function $$F(x, y)$$ such that $$\frac{\partial F(x, y)}{\partial x} = M(x, y) + N(x, y)\frac{dy}{dx}$$ Hence, $$\frac{\partial F(x, y)}{\partial x} = 0$$ The text says that the above equation imply that $$F(x, y) = \text{Const}$$ But here are my doubts 1) Let's assume the above equation is true, so does it mean $$F(x, y)$$ has no way to be a function of x if the original equation is exact? 2) What about if $$F(x,y) = g(y)$$, in this case, we don't know the exact form of y, but we know that y is depending on x, so can we say $$F(x,y)=g(y)$$ is implicitly depending on x? If it is true, how can we conclude that $$F(x, y) = \text{Const}$$ instead of some functions of y? Well, you might find what I am asking is vague. What I actually means is if $$F(x,y)=g(y)$$, so can we safely say that $$\frac{\partial F(x, y)}{\partial x} = \frac{\partial g(y)}{\partial x} = 0$$ It is quite confusing to use the term 'implicitly'! Because we do know that y=y(x), so how come we put $$\frac{\partial g(y)}{\partial x} = 0$$ instead of $$\frac{\partial g(y)}{\partial y}\frac{dy}{dx} = 0$$ ???