Chapter 2.6, Problem 17P

Assume that Eq. $\left(6\right)$ meets the requirements of Theorem $\mathrm{2.6.1}$ in a rectangle $R$ and is therefore exact. Show that a possible function $\psi \left(x,y\right)$ is $\psi (x,y)={\displaystyle {\int }_{x_0}^{x}M(s,y_0)ds}+{\displaystyle {\int }_{y_0}^{y}N(x_0,t)dt}$, where $\left(x_0,y_0\right)$ is a point in $R$.