B

A

D

2/3 nhân 5/2

a.10/6         b.5/3       c.4/15     d.15/4

25m9cm =     m 

a.25,09      b.259      c.2,59    d.2509

1/10 chia 2/5

a.5/2     b.2/50     c.2/5    d.1/4

a: \(A=\dfrac{15}{4}-\dfrac{9}{4}=\dfrac{6}{4}=\dfrac{3}{2}\)

b: \(B=\dfrac{-3}{2}\cdot\dfrac{7-5\cdot4}{3}=\dfrac{-13}{-2}=\dfrac{13}{2}\)

c: \(C=\dfrac{-21}{10}\left(1-\dfrac{3}{4}\right)-\dfrac{3}{4}\)

\(=\dfrac{-21}{10}\cdot\dfrac{1}{4}-\dfrac{3}{4}\)

\(=\dfrac{-21}{40}-\dfrac{30}{40}=\dfrac{-51}{40}000000997\)

d: \(D=\dfrac{-2}{3}+\dfrac{-12}{5}-3=\dfrac{-10-36-45}{15}=\dfrac{-91}{15}\)

a, \(x\) + 99: 3 = 55

    \(x\) + 33 = 55

     \(x\)        = 55 - 33

      \(x\)       = 22

b, (\(x\) - 25):15 = 20

       \(x\) - 25  = 20 x 15

       \(x\) - 25  = 300

        \(x\)         = 300 + 25

          \(x\)        = 325

c, (3\(x\) - 15).7 = 42

      3\(x\) - 15 = 42:7

       3\(x\) - 15 = 6

        3\(x\)         = 6 + 15

         3\(x\)         = 21

            \(x\)         = 21: 3

             \(x\)         = 7

d, (8\(x\) - 16).(\(x\) -5) = 0

       \(\left[{}\begin{matrix}8x-16=0\\x-5=0\end{matrix}\right.\)

         \(\left[{}\begin{matrix}8x=16\\x=5\end{matrix}\right.\)

          \(\left[{}\begin{matrix}x=16:8\\x=5\end{matrix}\right.\)

           \(\left[{}\begin{matrix}x=2\\x=5\end{matrix}\right.\)

Vậy \(x\) \(\in\) {2; 5}

`6/5 + 3/5 + ... =2`

`=> 2- 6/5 -3/5= 10/5 - 6/5 -3/5=1/5`

Mà `1/5=2/10`

`->B`

` 6/5+3/5+...=2`

`9/5+...=2`

`2-9/5=...`

`10/5-9/5=1/5`

`1/5=2/10`

=> Chọn đáp án `B`

Ta có: \(\dfrac{6}{5}+\dfrac{3}{5}+...=2\)

\(\dfrac{9}{5}+...=2\)

\(...=2-\dfrac{9}{5}\)

\(...=\dfrac{10}{5}-\dfrac{9}{5}\)

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

Mà \(\dfrac{1}{5}=\dfrac{2}{10}\)

`->` Đáp án B.

Chọn B

7:

=>\(\dfrac{x+10-2x+10}{x-5}>=0\)

=>(-x+20)/(x-5)>=0

=>(x-20)/(x-5)<=0

=>5<x<=20

=>a=5; b=20

2a+b=30