OJ Board

Hi,I want to say something first...my English is pool...
But I got A.C. and also think the problem is beautiful and clear enough...

if r1=r2,
then r1+r2=R=t/2
so R^2=t^2/4
the gray area=(t^2/4)*pi-2*(t^2/16)*pi=(t^2/8)*pi

else r1!=r2
then (r1+r2)^2=(r1-r2)^2+(t/2)^2
we can get 4*r1*r2=(t/2)^2
by ...