A

Chọn A

a,

\(y:\dfrac{2}{5}=12\)

\(y=12\times\dfrac{2}{5}\)

\(y=\dfrac{24}{5}=4,8\)

b,

\(\dfrac{11}{10}+\dfrac{4}{5}\times\dfrac{3}{4}=\dfrac{11}{10}+\dfrac{12}{20}=\dfrac{22}{20}+\dfrac{12}{20}=\dfrac{34}{20}=\dfrac{17}{10}\)

a) Ta có: \(\frac{x}{9}=\frac{-12}{27}\)

=> \(27.x=-12.9\)

=> \(27x=-108\)

=> \(x=108:27\)

=>\(x=4\)

bài 1 :

\(\frac{5}{12}\) + \(\frac{1}{3}\) = \(\frac{5}{12}\) + \(\frac{4}{12}\) = \(\frac{5+4}{12}\) = \(\frac{9}{12}\) = \(\frac{3}{4}\)

1 - \(\frac{3}{4}\) = \(\frac{4}{4}\) - \(\frac{3}{4}\) = \(\frac{4-3}{4}\) = \(\frac{1}{4}\)

2 x \(\frac{3}{11}\) = \(\frac{2x3}{11}\) = \(\frac{6}{11}\)

\(\frac{8}{7}\): 4 = \(\frac{8}{7x4}\) = \(\frac{8}{28}\) = \(\frac{2}{7}\)

bài 2 :

\(\frac{2}{5}\) : y = \(\frac{1}{3}\) 

           y = \(\frac{2}{5}\) : \(\frac{1}{3}\)

           y = \(\frac{6}{5}\) 

y : \(\frac{7}{11}\) = 22

y             = 22 x \(\frac{7}{11}\)

y             = 14

ai thấy đúng tk nha

5/12 + 1/3 = 3/4

1 - 3/4 = 1/4

2 x 3/11 = 6/11

8/7 : 4 = 2/7

Tìm y

2/5 : y = 1/3

y =2/5 : 1/3

y = 6/5

y : 7/11 = 22

y = 22 x 7/11

y = 2/7

\(\frac{5}{12}+\frac{1}{3}=\frac{5}{12}+\frac{4}{12}=\frac{9}{12}\)\(=\frac{3}{4}\)

\(1-\frac{3}{4}=\frac{4}{4}-\frac{3}{4}=\frac{1}{4}\)

\(2\times\frac{3}{11}=\frac{6}{11}\)

\(\frac{8}{7}\div4=\frac{8}{7\times4}=\frac{2}{7}\)

Ta co \(\frac{2}{5}\div y=\frac{1}{3}\Rightarrow y=\frac{2}{5}\div\frac{1}{3}\Rightarrow y=\frac{6}{5}\)

         \(y:\frac{7}{11}=22\Rightarrow y=22\times\frac{7}{11}\Rightarrow y=14\)

NHO K CHO MINH NHE

\(a.\)\(y-\frac{11}{3}=\frac{12}{5}\)

\(y=\frac{12}{5}+\frac{11}{3}\)

\(y=\frac{91}{15}\)

\(b.2-y=\frac{15}{6}\)

            \(y=2-\frac{15}{6}\)

           \(y=-\frac{1}{2}\)

\(c.y\times2\frac{2}{3}=\frac{4}{9}\)

 \(y=\frac{4}{9}:2\frac{2}{3}\)

\(y=\frac{1}{6}\)

\(d.y:1\frac{2}{5}=\frac{21}{10}\)

 \(y=\frac{21}{10}\times1\frac{2}{5}\)

 \(y=\frac{147}{5}\)

Ta có: \(\frac{y}{3}+y\cdot\frac{2}{3}=\frac{y}{12}+\frac{11}{6}\)

=> \(\frac{y}{3}+\frac{2y}{3}=\frac{y}{12}+\frac{22}{12}\)

=> \(\frac{y+2y}{3}=\frac{y+22}{12}\)

=> \(\frac{3y}{3}=\frac{y+22}{12}\)

=> \(y=\frac{y+22}{12}\)

=> y + 22 = 12y

=> y - 12y = 22

=> -11y = 22

=> y = 22 : (-11) = -2

Vậy y = -2

Ta có :

\(\frac{y+2}{11}+\frac{y+2}{12}+\frac{y+2}{13}=\frac{y+2}{14}+\frac{y+2}{15}\)

\(\Leftrightarrow\)\(\frac{y+2}{11}+\frac{y+2}{12}+\frac{y+2}{13}-\frac{y+2}{14}-\frac{y+2}{15}=0\)

\(\Leftrightarrow\)\(\left(y+2\right)\left(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}\right)=0\)

Vì \(\left(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}\right)\ne0\)

\(\Rightarrow\)\(y+2=0\)

\(\Rightarrow\)\(y=-2\)

Vậy \(y=-2\)

Chỗ trong ngoặc mk nhầm nhé phải là :

\(\left(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}-\frac{1}{14}-\frac{1}{15}\right)\) mới đúng sorry nha :')

ừ cảm ơn

A)

13 - 12 + 11 + 10 - 9 + 8 - 7 - 6 + 5 - 4 + 3 + 2 - 1

= 13 - ( 12 + 1 ) - ( 11 + 2 ) - ( 10 + 3 ) - ( 9 + 4 ) - ( 8 + 5 ) + ( 7 + 6 )

= 13  -      13     -       13     -       13      -     13      -      13     +      13

=        0             -                 0                -               0               +      13

= 13

13