(a+b)^2=6 -> a^2+b^2+2ab=6 (1)
(a-b)^2=8 -> a^2+b^2-2ab=8 (2)
(1)+(2)得 2a^2+2b^2=14 -> a^2+b^2=7
(1)-(2)得 4ab=-2 -> ab=-1/2