因为 (a - b)/ab = 4
所以 a - b = 4ab
所以 (a - 2ab - b)/(2a - 2b + 7ab)
= [(a - b) - 2ab]/[2(a - b) + 7ab]
= (4ab - 2ab)/(8ab + 7ab)
= 2ab/15ab
= 2/15