`7/12 + 3/4 xx 2/9`

`= 7/12 + 1/6`

`= 9/12`

_________________

`8/9 - 4/15 : 2/5`

`= 8/9 - 2/3`

`= 2/9`

giúp me 

a) _{^{\(\dfrac{7}{12}+\dfrac{3}{4}\times\dfrac{2}{9}=\dfrac{7}{12}+\dfrac{1}{6}=\dfrac{7}{12}+\dfrac{2}{12}=\dfrac{3}{4}\)}}

b) _{^{\(\dfrac{8}{9}-\dfrac{4}{15}:\dfrac{2}{5}=\dfrac{8}{9}-\dfrac{4}{15}\times\dfrac{5}{2}=\dfrac{8}{9}-\dfrac{2}{3}=\dfrac{8}{9}-\dfrac{6}{9}=\dfrac{2}{9}\)}}

Câu 9: A

Câu 10: C

Câu 11: C

Câu 12: A

Câu 13; B

Câu 14: C

 A

 C

C

 A

 B

C

Câu 8: A

Câu 7: B

Câu 6: B

Câu 5: A

25B

24B

23B

21A
22C

`3/2:x=3/4`

`x=3/2:3/4`

`x=3/2xx4/3`

`x=2`

hi

3/2 : x =3/4

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

\(x=2\)

`(2x^2 +5)(3x-2)=0`

`<=> 2x^2 +5=0` hoặc `3x-2=0`

`<=> 2x^2 =-5` (vô lí vì `2x^2 ≥0 ∀x`) hoặc `x=2/3`

vậy pt có tập nghiệm `S={2/3}`

\(\left(2x^2+5\right)\left(3x-2\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}2x^2=-5\\3x=2\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x^2=-10\left(l\right)\\x=\dfrac{2}{3}\left(n\right)\end{matrix}\right.\)

Vậy \(S=\left\{\dfrac{2}{3}\right\}\)

130

=130

200 - 120 + 50 = 80 + 50 = 130

1300

1300

1300

Câu 2 :

a. \(n_C=\dfrac{3.6}{12}=0,3\left(mol\right)\)

\(n_{O_2}=\dfrac{4.48}{22,4}=0,2\left(mol\right)\)

Ta thấy : 0,3 > 0,2 => C dư , O₂ đủ

PTHH : C + O₂ -> CO₂

            0,2   0,2     0,2

b. \(m_{CO_2}=0,2.44=8,8\left(g\right)\)

c.\(m_{O_2\left(dư\right)}\left(0,3-0,2\right).32=3,2\left(g\right)\)