已知：a+b=6,（ab）的负一次方=-1/7,

a-b/3ab×（a²-b²/a²+2ab+b²）负一次方
=(a-b)/3ab×(a+b)²/(a+b)(a-b)
=(a+b)/3ab
=(a+b)×(ab)的(-1)次方/3
=6×(-1/7)/3
=-2/7