a. x²=16.4

    x²=64

vì x<0 => x= -8

b: Tỉ số giữa 48 giờ và 3,2 giờ là:

48:3,2=15:1

c: \(\dfrac{-36}{7}\cdot\dfrac{2}{3}=\dfrac{-72}{21}=\dfrac{-24}{7}\)

\(a.\dfrac{1}{3}+\dfrac{4}{5}-\left(\dfrac{4}{5}+\dfrac{5}{8}\right)\)

\(=\dfrac{1}{3}+\dfrac{4}{5}-\dfrac{4}{5}-\dfrac{5}{8}\)

\(=\dfrac{1}{3}+\left(\dfrac{4}{5}-\dfrac{4}{5}\right)-\dfrac{5}{8}\)

\(=\dfrac{1}{3}-\dfrac{5}{8}\)

=\(-\dfrac{7}{24}\)

\(b.8\dfrac{4}{9}-\left(4\dfrac{2}{7}+5\dfrac{4}{9}\right)\)

\(=8\dfrac{4}{9}-4\dfrac{2}{7}-5\dfrac{4}{9}\)

\(=\left(8\dfrac{4}{9}-5\dfrac{4}{9}\right)-4\dfrac{2}{7}\)

\(=\left(8+\dfrac{4}{9}-5-\dfrac{4}{9}\right)-4-\dfrac{2}{7}\)

\(=\left[8-5+\left(\dfrac{4}{9}-\dfrac{4}{9}\right)\right]-4-\dfrac{2}{7}\)

\(=3-4-\dfrac{2}{7}\)

\(=-1-\dfrac{2}{7}\)

\(=-\dfrac{9}{7}\)

\(c.\left(-\dfrac{5}{7}\right).\dfrac{2}{11}+\left(-\dfrac{5}{7}\right).\dfrac{9}{11}+1\dfrac{5}{7}\)

\(=\left(-\dfrac{5}{7}\right).\dfrac{2}{11}+\left(-\dfrac{5}{7}\right).\dfrac{9}{11}+1+\left(-\dfrac{5}{7}\right).\left(-1\right)\)

\(=\left(-\dfrac{5}{7}\right).\left[\dfrac{2}{11}+\dfrac{9}{11}+\left(-1\right)\right]+1\)

\(=\left(-\dfrac{5}{7}\right).0+1=1\)

Ta có: 

`7/(-5) = (-7)/5 < -1`

`-1 < (-2)/5 < 0`

`0 < (-4)/(-5) = 4/5`

Thứ tự giảm dần là:  `4/5; (-2)/5; 7/(-5)`

\(\dfrac{2}{3}+\dfrac{5}{6}+\dfrac{9}{10}+\dfrac{14}{15}+\dfrac{20}{21}+\dfrac{27}{28}+\dfrac{35}{36}+\dfrac{44}{45}\\ =\left(1-\dfrac{1}{3}\right)+\left(1-\dfrac{1}{6}\right)+\left(1-\dfrac{1}{10}\right)+\left(1-\dfrac{1}{15}\right)+\left(1-\dfrac{1}{21}\right)+\left(1-\dfrac{1}{28}\right)+\left(1-\dfrac{1}{36}\right)+\left(1-\dfrac{1}{45}\right)\\ =8-\left(\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+\dfrac{1}{15}+\dfrac{1}{21}+\dfrac{1}{28}+\dfrac{1}{36}+\dfrac{1}{45}\right)\\ =8-\left(\dfrac{2}{6}+\dfrac{2}{12}+\dfrac{2}{20}+\dfrac{2}{30}+\dfrac{2}{42}+\dfrac{2}{56}+\dfrac{1}{72}+\dfrac{1}{90}\right)\\ =8-\left(\dfrac{2}{2.3}+\dfrac{2}{3.4}+\dfrac{2}{4.5}+\dfrac{2}{5.6}+\dfrac{2}{6.7}+\dfrac{2}{7.8}+\dfrac{2}{8.9}+\dfrac{2}{9.10}\right)\\ =8-2\left(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\right)\\ =8-2\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\right)\\ =8-2\left(\dfrac{1}{2}-\dfrac{1}{10}\right)=8-2.\dfrac{2}{5}=8-\dfrac{4}{5}=\dfrac{36}{5}\)

he nho

\(a.\dfrac{2}{3}+\dfrac{4}{3}:\dfrac{-2}{3}=\dfrac{2}{3}+\left(-2\right)=\dfrac{-4}{3}\)

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

\(c.\dfrac{-3}{5}.\dfrac{4}{7}+\dfrac{3}{7}.\dfrac{-3}{5}+\dfrac{3}{5}\\ =\dfrac{-3}{5}\left(\dfrac{4}{7}+\dfrac{3}{7}\right)+\dfrac{3}{5}\\ =\dfrac{-3}{5}+\dfrac{3}{5}=0\)

a) \(\dfrac{2}{5}+\dfrac{4}{3}:\dfrac{-2}{3}\)

\(=\dfrac{2}{5}+\dfrac{4}{3}.\dfrac{-3}{2}\)

\(=\dfrac{2}{5}+-2\)

\(=\dfrac{2}{5}+\dfrac{-10}{5}\)

\(=\dfrac{-8}{5}\)

Gọi số học sinh trong lớp là a(học sinh)

Suy ra số học sinh ở ngoài là $\dfrac{a}{5}$(học sinh)

Sau khi thêm hai học sinh vào lớp, ta có : 

$\dfrac{a}{5} - 2 = \dfrac{1}{7} ( a + 2)$

$\Rightarrow a = 40$

Suy ra : 

Số học sinh là $40 + \dfrac{40}{5} = 48$(học sinh)

a, - \(\dfrac{2}{5}\) + \(\dfrac{4}{5}\).\(x\) = \(\dfrac{3}{5}\)

               \(\dfrac{4}{5}\).\(x\) = \(\dfrac{3}{5}\)+ \(\dfrac{2}{5}\)

                 \(\dfrac{4}{5}\).\(x\) = 1

                      \(x\) = \(\dfrac{5}{4}\)

b, - \(\dfrac{3}{7}\) - \(\dfrac{4}{7}\): \(x\) = \(\dfrac{2}{5}\)

              \(\dfrac{4}{7}\): \(x\) = - \(\dfrac{3}{7}\) - \(\dfrac{2}{5}\)

                \(\dfrac{4}{7}\): \(x\) = - \(\dfrac{29}{35}\)

                  \(x\) = \(\dfrac{4}{7}\): (- \(\dfrac{29}{35}\) )

                  \(x\) = - \(\dfrac{20}{29}\)

c, \(\dfrac{4}{7}\).\(x\) + \(\dfrac{2}{3}\) = - \(\dfrac{1}{5}\)

     \(\dfrac{4}{7}\).\(x\)         = -\(\dfrac{1}{5}\) - \(\dfrac{2}{3}\)

      \(\dfrac{4}{7}\).\(x\)       = - \(\dfrac{13}{15}\)

           \(x\)     = - \(\dfrac{13}{15}\): \(\dfrac{4}{7}\)

            \(x\)    = - \(\dfrac{91}{60}\)