Answer :
[tex]a-side\ length\ of\ the\ square\\r=\frac{1}{2}a-radius\ length\ of\ the\ circle\\\\A_{\fbox{}}=a^2\\\\A_O=\pi r^2=\pi\left(\frac{1}{2}a\right)^2=\frac{1}{4}\pi a^2\\\\Probability:\frac{\frac{1}{4}\pi a^2}{a^2}=\frac{1}{4}\pi\approx\frac{1}{4}\cdot3.14=0.785[/tex]