red disk area

-1-

-2-

  

13^2=12^2+(R−r)2

↔

25=(R—r)2

⇒

[R−r=5 (1)

R+r=13 (2)

from

(1)+(2)

 2R=18

 R=9

red disk  area

=pi*9^2=81pi

-3-

-4-