S=π r2

1) КВАДРАТ 

    R= a / sqrt2     r =a / 2

S1 /S2 = π (a / sqrt2)^2   /  π (a/2)^2 =  2 

2) 12-угольник

  R

S1 /S2 =( π (  \frac{a}{2 sin(\frac{360*}{2*12}) }[/tex] )^2 ) /   π   \frac{a}{2 tg(\frac{360}{2*12})}[/tex] = tg 15 / sin 15