3ab一[2a - (2ab - 2b ) + 3]
=3ab - [2a - 2ab + 2b + 3]
= 3ab - 2a + 2ab - 2b - 3
= 5ab - 2(a + b) - 3
当 ab = 3 ,a + b = 4时,
原式＝5×3 - 2×4 －3 ＝ 4