 Define the side $$a$$ of a square of apexes $$ABCD$$.
 Two arcs $$P$$ and $$Q$$ are drawn centered, respectively, in $$B$$ and $$D$$. Both measure $$90^\circ$$, start in $$A$$ and end in $$C$$. Find the length of the arcs $$P$$ and $$Q$$.
 Determine the area inside the square and out of the diagram that is bounded by the arcs $$P$$ and $$Q$$.
See development and solution
Development:
 We define the side of the square as $$a=10$$.

Both are arcs of $$90^\circ$$ of circumferences, of radius $$10$$. And so, they will have a length of the quarter of the perimeter of the circumference of radius $$10$$: $$$l_p=l_q=\dfrac{2\pi\cdot r}{4}$$$ $$$l_p=l_q=5\pi$$$
 We first find the area of one of the two areas that are inside the square and out of the diagram formed by the arcs $$p$$ and $$q$$. This area will take as an area the difference between the area of the square and the area of a sector of $$90^\circ$$ of the circle of radius $$10$$.
$$$\mbox{Area} \ ACD = \mbox{Area} \ ABCD  \mbox{Area sector} \ BCA$$$ $$$A_{ACD}=100\dfrac{\pi \cdot 10^2}{4}=21,4$$$ $$$A_{total}=A_{ACD}+A_{ACB}=2\cdot A_{ACD}=42,8$$$
Solution:
 $$a=10$$
 $$l_p=l_q=5\pi$$
 $$A_{BLUE}=42,8$$