11

a , x+6=49-3-13

x+6=a

x=a-6

x=n

b,231-(x-6)=n

x-6=231-n

x-6=a

x=a+6

x=m

B1:

a)49-3.(x+6)=13

3.(x+6)=49-13

3.(x+6)=36

x+6=36/3

x+6=12

x=12-6

x=6

Bài 1:

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

=\(\dfrac{40}{60}\) + \(\dfrac{45}{60}\) - \(\dfrac{24}{60}\)

= \(\dfrac{61}{60}\)

b; \(\dfrac{12}{13}\) x \(\dfrac{3}{4}\)  - \(\dfrac{7}{13}\)

= \(\dfrac{9}{13}\) - \(\dfrac{7}{13}\)

= \(\dfrac{2}{13}\)

c; \(\dfrac{15}{17}\) : \(\dfrac{19}{34}\) - \(\dfrac{17}{19}\)

= \(\dfrac{15}{17}\) x \(\dfrac{34}{19}\) - \(\dfrac{17}{19}\)

= \(\dfrac{30}{19}\) - \(\dfrac{17}{19}\)

= \(\dfrac{13}{19}\)

Bài 2: 

a; \(\dfrac{x}{5}\) x \(\dfrac{3}{7}\) = \(\dfrac{9}{35}\)

     \(\dfrac{x}{5}\)       = \(\dfrac{9}{35}\) : \(\dfrac{3}{7}\)

      \(\dfrac{x}{5}\)      = \(\dfrac{3}{5}\)

       \(x\)      = \(\dfrac{3}{5}\) x 5

       \(x\)      = 3

Bài 2

b; \(\dfrac{5}{12}\) : \(\dfrac{6}{x}\) = \(\dfrac{1}{2}\)

             \(\dfrac{6}{x}\) = \(\dfrac{5}{12}\) : \(\dfrac{1}{2}\)

             \(\dfrac{6}{x}\) = \(\dfrac{5}{6}\)

              \(x\) = 6 : \(\dfrac{5}{6}\)

              \(x=\dfrac{36}{5}\)

1) = \(\frac{3}{5}\)

2) =\(\frac{6}{7}\)

3)\(\frac{9}{13}\)

4)\(\frac{4}{13}\)

a) -11x + 9 = 130

-11x = 121

x = -11

b) -12x = -15.(-4) - 12 

-12x = 48

x = -4

c) -5x + 5 = -3 .(-8) = 6 ( đéo hiểu đề kiểu j ) 

a) \(\left(-11\right)x+9=130\Rightarrow\left(-11\right)x=130-9=121\)

\(\Rightarrow x=\left(-121\right):\left(-11\right)\Leftrightarrow x=-11\)

b) \(\left(-12\right)x=\left(-15\right)\left(-4\right)\left(-12\right)\Rightarrow\left(-12\right)x=\left(-12\right).60\Rightarrow x=60\)

c) P/s : sai đề \(\left(-3\right)\left(-8\right)=24\ne6\)

nhầm câu c:

c,(-5).x+5=(-3).(-8)+6

Đáp án cần chọn là: A

Ta có(−9).(−12)−(−13).6=108−(−78)=108+78=186

Phương pháp giải:

Tính nhẩm các số rồi điền kết quả vào chỗ trống.

Lời giải chi tiết:

Tính nhẩm:

A  =\(\frac{2}{5}\times\frac{4}{7}+\frac{3}{5}\times\frac{4}{7}\)  B = \(\frac{17}{13}\times\frac{12}{17}-\frac{4}{13}\times\frac{12}{17}\)   C = \(\frac{9}{14}:\frac{6}{7}+\frac{5}{14}:\frac{6}{7}\)  D =\(\frac{9}{12}:\frac{7}{9}\times\frac{12}{19}\)

=  \(\frac{4}{7}\times\left(\frac{2}{5}+\frac{3}{5}\right)\)              =\(\left(\frac{17}{13}-\frac{4}{13}\right)\times\frac{12}{17}\)                = \((\frac{9}{14}+\frac{5}{14}):\frac{6}{7}\)         = \(\frac{9}{12}\times\frac{9}{7}\times\frac{12}{19}\)

=\(\frac{4}{7}\times1\)                                     = \(1\times\frac{12}{17}\)                                       = \(1:\frac{6}{7}\)                               = \(\frac{9\times9\times12}{12\times7\times19}\)

=\(\frac{4}{7}\)                                              =  \(\frac{12}{17}\)                                                =\(\frac{7}{6}\)                                    = \(\frac{9\times9}{7\times19}=\frac{81}{133}\)

   -  

a) \(\dfrac{3}{8}+\dfrac{7}{12}+\dfrac{10}{16}+\dfrac{10}{24}\)

\(=\dfrac{3}{8}+\dfrac{7}{12}+\dfrac{5}{8}+\dfrac{5}{12}\)

\(=\left(\dfrac{3}{8}+\dfrac{5}{8}\right)+\left(\dfrac{7}{12}+\dfrac{5}{12}\right)\)

\(=1+1\)

\(=2\)

b) \(\dfrac{4}{6}+\dfrac{7}{13}+\dfrac{17}{9}+\dfrac{19}{13}+\dfrac{1}{9}+\dfrac{14}{6}\)

\(=\dfrac{2}{3}+\dfrac{7}{13}+\dfrac{17}{9}+\dfrac{19}{13}+\dfrac{1}{9}+\dfrac{7}{3}\)

\(=\left(\dfrac{2}{3}+\dfrac{7}{3}\right)+\left(\dfrac{7}{13}+\dfrac{19}{13}\right)+\left(\dfrac{17}{9}+\dfrac{1}{9}\right)\)

\(=3+2+2\)

\(=7\)

c) \(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}\)

\(=\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}\)

\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\)

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

\(=\dfrac{6}{7}\)

5/9 x 3/8 = 15/72 = 5/24
5/6 + 9/12 : 13/8 = 5/6 + 9/12 x 8/13 = 5/6 + 6/13 = 65/78 + 36/78                                                                                         = 101/78

5/9 x 3/8

= 5x3/9x8

= 15/72=5/26

5/6 + 9/12 : 13/8

= 5/6 + 72/156

= 91/78