smc

Let $a$ be a positive number. Consider the set $S$ of all points whose rectangular coordinates $(x,y)$ satisfy all of the following conditions: $$\begin{gathered} \text{(i) }\frac{a}{2}\le x\le 2a\text{(ii) }\frac{a}{2}\le y\le 2a\text{(iii) }x+y\ge a \\ \text{(iv) }x+a\ge y\text{(v) }y+a\ge x \end{gathered}$$ The boundary of set $S$ is a polygon with