解：大圓的直徑為?$a+2b，$?因此半徑為?$\frac {a+ 2b}2$?
面積為：?$ S_{大圓} = \pi (\frac {a+ 2b}2 )^2 = \pi (\frac {a^2 + 4ab + 4b^2}4 ) = \frac {\pi (a^2 + 4ab + 4b^2)}4 $?
三個小圓的直徑分別為?$a、$??$b、$??$b$?
因此半徑分別為?$\frac {a}2、$??$\frac 2、$??$\frac 2$?
面積分別為：?$ S_{小圓1} = \pi (\frac {a}2 )^2 = \frac {\pi a^2}4，$??$S_{小圓2} = \pi (\frac 2 )^2 = \frac {\pi b^2}4 $?
?$S_{小圓3} = \pi (\frac 2 )^2 = \frac {\pi b^2}4 $?
三個小圓的總面積為：?$ S_{小圓總} = \frac {\pi a^2}4 + \frac {\pi b^2}4 + \frac {\pi b^2}4 = \frac {\pi (a^2 + 2b^2)}4 $?
∴?$S_{剩下} = S_{大圓} - S_{小圓總} $?
?$= \frac {\pi (a^2 + 4ab + 4b^2)}4 - \frac {\pi (a^2 + 2b^2)}4 = \frac {\pi ( 4ab + 2b^2)}4 $?
