https://vn.answers.yahoo.com/question/index?qid=20100907070553AAEawEd

lời giải rõ ràng rồi đó bạn ^_~

\(\frac{15-\frac{15}{7}-\frac{15}{12}}{3-\frac{3}{7}-\frac{3}{12}}\)

\(=\frac{15\left(1-\frac{1}{7}-\frac{1}{12}\right)}{3\left(1-\frac{1}{7}-\frac{1}{12}\right)}\)

\(=\frac{15}{3}\)

K  nha 

\(15-\frac{15}{7}-\frac{15}{12}=11\frac{17}{28}\)

\(=\frac{325}{28}\)

\(3-\frac{3}{7}-\frac{3}{13}=2\frac{31}{91}\)

\(=\frac{213}{91}\)

làm từng bước bạn ơi !

a )   ( -1/6 + 5/12 ) + 7/12

= -1/6 + ( 5/12 + 7/12 ) 

= -1/6 + 12/12

= -2/12 + 12/12 

=        -10/12

=        -5/6

\(\frac{12}{15}\)+  \(\frac{4}{5}\)= \(\frac{8}{5}\)

\(\dfrac{3}{5}.\dfrac{5}{7}+\dfrac{12}{20}.\dfrac{2}{14}+\dfrac{9}{15}.\dfrac{1}{7}\)

\(=\dfrac{3}{7}+\dfrac{3}{35}+\dfrac{3}{35}\)

\(=\dfrac{3}{7}+\dfrac{6}{35}\)

\(=\dfrac{15}{35}+\dfrac{6}{35}\)

\(=\dfrac{21}{35}\)

\(=\dfrac{3}{5}\)

...=-15-9=-24

...=4+7=11

...=-12-13=-25

...=13+10=23

...=-4-7=-11

...=-9+13=4

...=12+0=12

...=0-7=-7

...=0+3=3

...=15-17=-2

...=-6

...=-23

...=15-15=0

-15-(+9) = -24

4-(-7) = 11

-12+(-13) = -25

-(-13)-(-10) = 23

-4-(+7) = -11

(-9)-(-13) = 4

-(-12)+0 = 12

0-(+7) = -7

0-(-3) = 3

15-17 = -2

-18+12 = -6

-5-18 = -23

-(-15)+(-15) = 0