그린 정리

(a) Expressing the left hand side as an iterated integral, with $y$ as the inner integration variable, we have

$$\begin{aligned} \iint _R\frac{\partial f}{\partial y}(x,y)\ \mathrm{d}x\,\mathrm{d}y &=\int_0^1\mathrm{d}x\left[\int_0^1\mathrm{d}y\ \frac{\partial f}{\partial y}(x,y)\right] \\ &=\int_0^1\mathrm{d}x\ \big[f(x,1)-f(x,0)\big] \\ &\text{by the fundamental theorem of calculus} \\ &=\int_0^1 f(x,1)\ \mathrm{d}x - \int_0^1 f(x,0)\ \mathrm{d}x \end{aligned}$$

(b) Define $F_1(x,y)=f(x,y)$ and $F_2(x,y)=0$. Then. by Green's theorem

$$\begin{aligned} \iint _R\frac{\partial f}{\partial y}(x,y)\ \mathrm{d}x\,\mathrm{d}y &=-\iint _R\Big[\frac{\partial F_2}{\partial x}(x,y) -\frac{\partial F_1}{\partial y}(x,y)\Big]\ \mathrm{d}x\,\mathrm{d}y \\ &=-\int_{\partial R}\big[F_1(x,y)\,\mathrm{d}x +F_2(x,y)\,\mathrm{d}y\big] \\ &=-\int_{\partial R}f(x,y)\,\mathrm{d}x \end{aligned}$$

The boundary of $R$, oriented counterclockwise, is the union of four line segments.

$$\begin{aligned} \\[-0.1in] &\text{$C_1$ from $(0,0)$ to $(1,0)$} \qquad\smash{  }\\ &\text{$C_2$ from $(1,0)$ to $(1,1)$} \\ &\text{$C_3$ from $(1,1)$ to $(0,1)$} \\ &\text{$C_4$ from $(0,1)$ to $(0,0)$} \\[-0.1in] \end{aligned}$$

Now $x$ is constant on $C_2$ and $C_4$ so that

$$\int_{C_2}f(x,y)\,\mathrm{d}x = \int_{C_4}f(x,y)\,\mathrm{d}x =0$$

So, using $-C_3$ to denote the line segment from $(0,1)$ to $(1,1)$

$$\begin{aligned} \iint _R\frac{\partial f}{\partial y}(x,y)\ \mathrm{d}x\,\mathrm{d}y &=-\left[\int_{C_1}f(x,y)\,\mathrm{d}x+\int_{C_3}f(x,y)\,\mathrm{d}x\right]\\ &=\int_{-C_3}f(x,y)\,\mathrm{d}x-\int_{C_1}f(x,y)\,\mathrm{d}x \\ &=\int_0^1 f(x,1)\ \mathrm{d}x - \int_0^1 f(x,0)\ \mathrm{d}x \end{aligned}$$