Lời giải:
a. 

$\frac{7}{4}+\frac{5}{-6}+\frac{21}{8}=\frac{42}{24}+\frac{-20}{24}+\frac{63}{24}=\frac{85}{24}$

b.

$\frac{4}{9}+\frac{-7}{12}+\frac{8}{15}$

$=\frac{80}{180}+\frac{-105}{180}+\frac{96}{180}=\frac{71}{180}$

c.

$=\frac{-1}{3}+\frac{5}{6}+\frac{19}{12}+2$
$=\frac{-4}{12}+\frac{10}{12}+\frac{19}{12}+2=\frac{25}{12}+2=\frac{49}{12}$

bạn ơi , tính đầy đủ cho mình với

địt con mẹ mày lên

\(\dfrac{1}{3}\)-\(\dfrac{2}{15}\)+\(\dfrac{14}{15}\)=\(\dfrac{3}{15}\)-\(\dfrac{2}{15}\)+\(\dfrac{14}{15}\)

=\(\dfrac{13}{15}\)

\(\dfrac{3}{5}\)+4-\(\dfrac{6}{7}\)=\(\dfrac{3}{5}\)+\(\dfrac{20}{5}\)-\(\dfrac{6}{7}\)=\(\dfrac{23}{5}\)-\(\dfrac{6}{7}\)=\(\dfrac{161}{35}\)-\(\dfrac{30}{35}\)=\(\dfrac{131}{35}\)

\(\frac{2}{7}+\frac{5}{7}=\frac{7}{7}=1\)

\(\frac{8}{15}+\frac{6}{15}=\frac{14}{15}\)

\(\frac{2}{3}+\frac{3}{9}+\frac{4}{9}=\frac{6}{9}+\frac{3}{9}+\frac{4}{9}=\frac{9}{9}+\frac{4}{9}=\frac{13}{9}\)

1,14/15,13/9,

2/7 + 5/7 = 7/7 = 1

8/15 + 6/15 = 14/15

2/3 + 3/9 + 4/9 = 6/9 + 3/9 + 4/9 = 13/9

a; (15 - \(\dfrac{121}{18}\)) : \(\dfrac{297}{27}\) - \(\dfrac{17}{8}\) : \(\dfrac{51}{40}\)

     (\(\dfrac{270}{18}\) - \(\dfrac{121}{18}\)) : \(\dfrac{297}{27}\) - \(\dfrac{17}{8}\) x \(\dfrac{40}{51}\)

 =   \(\dfrac{149}{18}\) : \(\dfrac{297}{27}\) - \(\dfrac{5}{3}\)

= \(\dfrac{149}{18}\) x \(\dfrac{27}{297}\) - \(\dfrac{5}{3}\)

 = \(\dfrac{149}{198}\) - \(\dfrac{5}{3}\)

= \(\dfrac{149}{198}\) - \(\dfrac{330}{198}\)

= \(\dfrac{-181}{198}\)

b; (- 3,2) x (- \(\dfrac{15}{64}\)) + (0,8 - \(\dfrac{34}{15}\)): \(\dfrac{11}{3}\)

= (\(\dfrac{-16}{5}\)) x ( \(\dfrac{-15}{64}\)) + (\(\dfrac{4}{5}\) - \(\dfrac{34}{15}\)): \(\dfrac{11}{3}\)

= \(\dfrac{3}{4}\) + (\(\dfrac{4}{5}\) - \(\dfrac{34}{15}\)): \(\dfrac{11}{3}\)

= \(\dfrac{3}{4}\) + (\(\dfrac{12}{15}\) - \(\dfrac{34}{15}\)) : \(\dfrac{11}{3}\)

= \(\dfrac{3}{4}\) + \(\dfrac{-22}{15}\) : \(\dfrac{11}{3}\)

= \(\dfrac{3}{4}\) - \(\dfrac{22}{15}\) x \(\dfrac{3}{11}\)

= \(\dfrac{3}{4}\) - \(\dfrac{2}{5}\)

= \(\dfrac{15}{20}\) - \(\dfrac{8}{20}\)

= \(\dfrac{7}{20}\)

c;   \(\dfrac{6}{7}\) + \(\dfrac{5}{8}\): 5 - \(\dfrac{3}{16}\) x (- 2)²

=   \(\dfrac{6}{7}\) +  \(\dfrac{5}{8}\) x \(\dfrac{1}{5}\) - \(\dfrac{3}{16}\) x 4

=  \(\dfrac{6}{7}\) + \(\dfrac{1}{8}\) - \(\dfrac{3}{4}\)

=   \(\dfrac{48}{56}\) + \(\dfrac{7}{56}\) - \(\dfrac{42}{56}\)

= \(\dfrac{55}{56}\) - \(\dfrac{42}{56}\)

=  \(\dfrac{13}{56}\)

MSC : `60`

`-2/5=(-2.12)/(5.12)=-24/60`

`5/(-6)=-5/6=(-5.10)/(6.10)=-50/60`

`-7/12=(-7.5)/(12.5)=-35/60`

`8/(-15)=-8/15=(-8.4)/(15.4)=-32/60`

Thứ tự tăng dần : 

`-50/60;-35/60;-32/60;-24/60`

`#iuv`

k minh minh giai

* (-6 - 2) . (-6 + 2)

=    -8   .     -4

=           32

*(7 . 3 - 3) : (-6)

=      18    : (-6)

=            -3

k cho mik nhe

(-6-2).(-6+2)

=-8.(-4)

=32

(7.3-3):(-6)

=(21-3):(-6)

=18:(-6)

=-3

(-5+9).(-4)

=4.(-4)

=-16

72:(-6.2+4)

=72.(-12+4)

=72.8

-3.7-4.(-5)+1

=-21+20+1

=-1+1

=0

18-10:(+2)-7

=18-5-7

=13-7

=6

15:(-5).(-3)-8

=-3.(-3)-8

=9-8

=1

(6.8-10:5)+3.(-7)

=(48-2)+3.(-7)

=46+(-21)

=25

a) \(...=-\dfrac{1}{4}.\dfrac{4}{17}.\left(-\dfrac{63}{21}\right).\left(-\dfrac{7}{12}\right)\)

\(=-\dfrac{1}{17}.\dfrac{63}{21}.\dfrac{7}{12}\)

\(=-\dfrac{7}{68}\)

b) \(...=-\dfrac{2}{5}.\dfrac{4}{15}-\dfrac{3}{10}.\dfrac{4}{15}\)

\(=\dfrac{4}{15}\left(-\dfrac{2}{5}-\dfrac{3}{10}\right)\)

\(=\dfrac{4}{15}\left(-\dfrac{4}{10}-\dfrac{3}{10}\right)\)

\(=\dfrac{4}{15}.\left(-\dfrac{7}{10}\right)=-\dfrac{14}{75}\)

c) \(...=21-\dfrac{15}{4}:\left(\dfrac{9}{24}-\dfrac{4}{24}\right)\)

\(=21-\dfrac{15}{4}:\dfrac{5}{24}\)

\(=21-\dfrac{15}{4}.\dfrac{24}{5}\)

\(=21-3.6=3\)

d) \(...=\left(-\dfrac{3}{4}+\dfrac{2}{5}\right).\dfrac{7}{3}+\left(\dfrac{3}{5}-\dfrac{1}{4}\right).\dfrac{7}{3}\)

\(=\dfrac{7}{3}\left(-\dfrac{3}{4}+\dfrac{2}{5}+\dfrac{3}{5}-\dfrac{1}{4}\right)\)

\(=\dfrac{7}{3}\left(-\dfrac{3}{4}-\dfrac{1}{4}+\dfrac{2}{5}+\dfrac{3}{5}\right)\)

\(=\dfrac{7}{3}\left(-1+1\right)=0\)

a)(-0,25).4/17.(-3 5/21).(-7/12)

= -1/17 . (-68/21) . ( -7/21)

= -4/63

b) (-2/5).4/15+(-3/10).4/15

= 4/15. [-2/5 + (-3/10)]

= 4/15 . (-7/10)

= -14/75

c)21-3 3/4:(3/8-1/6)

= 21 - 15/4 : 5/24

= 21 - 18

= 3

d) (-3/4+2/5):3/7+(3/5+-1/4):3/7

= [(-3/4 + 2/5) + (3/5 + -1/4)] : 3/7

= [-7/20 + (-17/20)] : 3/7

= -6/5 : 3/7

= -14/5