花坛周长为40m
a + b = 40 / 2 = 20m
设花坛原面积为x .
a * b = x
b = 20 - a
(a - 4) ( b -2 ) = x + 8
(a - 4 ) ( 20 - a - 2 ) - 8 = x
得：a = 16,b = 4
验证：
16 * 4 = 64
( 16 - 4 ) ( 4 + 2 ) = x + 8
72 = x + 8
x = 64