Bài 1 :

a) `6/5+4/3*21/8-13/10`

=`6/5+7/2-13/10`

=`12/10+35/10-13/10`

=`(12+35-13)/10`

=`34/10`

=`17/5`

b)`(-11)/12*18/25+(-11)/12*7/25+11/12`

=`(-11)/12*18/25+(-11)/12*7/25+(-11)/12*(-1)`

=`(-11)/12*(18/25+7/25-1)`

=`(-11)/12*(1-1)`

=`(-11)/12*0`

=`0`

c)`12,89+27,11-43,65+(-56,35)`

=`12,89+27,11-43,65-56,35`

=`(  12,89+27,11 )-( 43,65+56,35 )`

=`40-100`

=`-60`

Bài 2 :

a) `x-3=2/3`

=>`x=2/3+3/5`

=>`x=19/15`

b) `1/4-(3/4-x)=2`

=>`3/4-x=1/4-2`

=>`3/4-x=(-7)/4`

=>`x=3/4-(-7/4)`

=>`x=10/4=5/2`

c)` (x+2)/-4=-9/(x+2)`

=>`(x+2)*(x+2)=-4*(-9)`

=>`(x+2)^2=36`

=>`(x+2)^2=6^2`

TH1 :

`x+2=6`

=>`x=6-2`

=>`x=4`

TH2:

`x+2=*6`

=>`x=-6-2`

=>`x=-8`

Vậy `x=4` hoặc `x=-8`