$\text{Bài 3:}$
`6 xx 9 - 4 : 2 + 3`
`= (6 xx 9 - 4) : (2 + 3)`
`= (54- 4) : 5`
`= 50 : 5`
`= 10`
-----------------------
$\text{Bài 4:}$
`a, 2011 + \underbrace{3+3+...+3}_{2011 chữ số 3}`
`= 2011 + 2011 xx 3`
`= 2011 xx 1 + 2011 xx 3`
`= 2011 xx (1 + 3)`
`= 2011 xx 4`
`= 8044`
-----------------------
`b, \underbrace{7+7+...+7}_{2011 chữ số 7} - \underbrace{5-5-...-5}_{2011 chữ số 5}`
`= 2011 xx 7 - 2011 xx 5`
`= 2011 xx (7 - 5)`
`= 2011 xx 2`
`= 4022`
