`13) 1/5. (-7/3 + 2/8) : 0,25`
`= 1/5. (-28/12 + 3/12) : 1/4`
`= 1/5. -25/12. 4`
`= -5/12. 4`
`= -5/3`
`14) 1/4 - (13/19 - (-2)/8) + (13/19 - 5/7)`
`= 1/4 - (13/19 + 1/4) + 13/19 - 5/7`
`= 1/4 - 13/19 - 1/4 + 13/19 - 5/7`
`= (1/4 - 1/4) + (-13/19 + 13/19) - 5/7`
`= 0 - 5/7`
`= -5/7`
`15) 9/24 + (-8)/(-14) - 12/32`
`= 3/8 + 4/7 - 3/8`
`= (3/8 - 3/8) + 4/7`
`= 0 + 4/7`
`= 4/7`
`16) | 2^2/18 + 3/9 |- 6/27`
`= | 4/18 + 1/3| - 2/9`
`= | 2/9 + 1/3| - 2/9`
`= 2/9 + 1/3 - 2/9` `**) |a| = a` nếu `a >= 0`
`= (2/9 - 2/9) + 1/3`
`= 0 + 1/3`
`= 1/3`
