12 + 21 x 12 + 12 x 34

= 12 x ( 1 + 12 + 34 )

= 12 x 45

= 540

12 + 21 x 12 + 12 x 34

= 12 x 1 + 21 x 12 + 12 x 34

= 12 x (1+21+34)

= 12 x 56

= 672

a) \(...=-\dfrac{12}{46}-\dfrac{15}{21}+\dfrac{3}{7}-\dfrac{7}{23}\)

\(=-\dfrac{12}{46}-\dfrac{7}{23}-\dfrac{15}{21}+\dfrac{3}{7}\)

\(=-\dfrac{6}{23}-\dfrac{7}{23}-\dfrac{5}{7}+\dfrac{3}{7}\)

\(=-\dfrac{13}{23}-\dfrac{2}{7}\)

\(=-\dfrac{13.7}{23.7}-\dfrac{2.23}{23.7}\)

\(=-\dfrac{91}{161}-\dfrac{46}{161}=-\dfrac{137}{161}\)

b) \(...=-21+\dfrac{7}{9}-\dfrac{3}{17}+\dfrac{25}{9}\)

\(=-21-\dfrac{3}{17}+\dfrac{7}{9}+\dfrac{25}{9}\)

\(=-21-\dfrac{3}{17}+\dfrac{32}{9}\)

\(=-\dfrac{3213}{153}-\dfrac{27}{153}+\dfrac{544}{153}=-\dfrac{2696}{153}\)

\(b,\left(15-x\right)+\left(x-12\right)=7-\left(-8-x\right)\)

\(\Leftrightarrow15-x+x-12=7+8+x\)

\(\Leftrightarrow3=15+x\)

\(\Leftrightarrow x=3-15\)

\(\Leftrightarrow x=-12\)

Vậy : \(x=-12\)

Mik sửa lại câu a cho bạn Đạt này

A. 34 + (9 - 21)= 3417 - (x + 3417)

<=>34 + (-12) = 3417 - x - 3417

<=>22 = -x

=> x = -22

Vậy x= -22

42 x 2 = 84

34 x2 = 68

12 x 3 = 36

21 x 3 = 63

42 x 2 = 84

34 x 2 = 68

12 x 3 = 36

21 x 3 = 63

k mk,mk k lại ^_-

130

\(-\dfrac{12}{21}\div\dfrac{34}{43}\)

\(=-\dfrac{12}{21}\times\dfrac{43}{34}\)

\(=-\dfrac{86}{119}\)

\(-\dfrac{12}{21}:\dfrac{34}{43}=-\dfrac{12}{21}.\dfrac{43}{34}=-\dfrac{2}{7}.\dfrac{43}{17}=-\dfrac{86}{119}\)

\(21+34+36-12\)

\(=\left(34+36\right)+\left(21-12\right)\)

\(=70+9\)

\(=79\)

$21+34+36-12$

$=\left(34+36\right)+\left(21-12\right)$

$=70+9$

$=79$