# This ODE $(\dot x,\dot y)=(f(x),g(x)y)$ has only one solution

I'm trying to solve this question:

Let $$f,g:\mathbb R\to \mathbb R$$ be continuous and Lipschitz. Prove the initial value problem:

$$(\dot x,\dot y)=(f(x),g(x)y)$$, $$x(t_0)=x_0$$, $$y(t_0)=y_0$$ has only one solution in every interval.

I've tried to use the Picard theorem without success. I think maybe I have to adapt this theorem for this case.

I really need help.

Any help is welcome.

Since the equation for $x$ does not involve $y$, you can solve it first as a single ODE: $$\dot x = f(x), \quad x(t_0)=x_0\tag1$$ Since $f$ is globally Lipschitz, the Picard theorem implies that the solution of (1) exists for all times, and is unique. Now that $x$ is a known function, the equation for $y$ turns into a linear ODE $$\dot y = g(x)y, \quad y(t_0)=y_0\tag2$$ Here $g(x)$ is a continuous function of $t$, being the composition of two continuous functions. It follows that the solution of (2) exists for all times, and is unique.