แก้โจทย์ A=(D^2-d^2)*pi/4*alpha/360^circ

$$A = ( D ^ { 2 } - d ^ { 2 } ) \cdot \frac { \pi } { 4 } \cdot \frac { \alpha } { 360 ^ { \circ } }$$

หาค่า A

$$A=\frac{\pi \alpha \left(D^{2}-d^{2}\right)}{1440}$$

หาค่า D

$$\left\{\begin{matrix}D=\sqrt{\frac{1440A+\pi \alpha d^{2}}{\pi \alpha }}\text{; }D=-\sqrt{\frac{1440A+\pi \alpha d^{2}}{\pi \alpha }}\text{, }&\left(\alpha <0\text{ or }A\geq -\frac{\pi \alpha d^{2}}{1440}\right)\text{ and }\left(\alpha >0\text{ or }A\leq -\frac{\pi \alpha d^{2}}{1440}\right)\text{ and }\alpha \neq 0\\D\in \mathrm{R}\text{, }&A=0\text{ and }\alpha =0\end{matrix}\right.$$