Giải thích các bước giải:
`2/3 .x = -21/27`
`=>x=-21/27:2/3`
`=>x=-21/27 . 3/2`
`=>x=-63/54`
`=>x=-7/6`
`-1 3/5 . x =1 1/15`
`=>-8/5 . x=16/15`
`=>x=16/15 : (-8)/5`
`=>x=16/15 . (-5)/8`
`=>x=-80/120`
`=>x=-2/3`
`1/3 + 1/2 :x =-4`
`=>1/2:x=1/3-(-4)`
`=>1/2:x=1/3+4`
`=>1/2:x=1/3+12/3`
`=>1/2:x=13/3`
`=>x=1/2:13/3`
`=>x=1/2. 3/13`
`=>x=3/26`
`3/4+1/4 :x = 2/5`
`=>1/4:x=2/5-3/4`
`=>1/4:x=8/20-15/20`
`=>1/4:x=-7/20`
`=>x=1/4:(-7)/20`
`=>x=1/4 . (-20)/7`
`=>x=-20/28`
`=>x=-10/14`
`=>x=-5/7`
