(4a^2 - 4a + 1) + (9b^2 + 12b + 4) = 0
(2a - 1)^2 + (3b + 2)^2 = 0
要使上式成立,必须：
2a - 1 = 0,3b + 2 = 0
即：a = 1/2,b = -2/3
因此,ab = -1/3