Bài 1.28 trang 19 SBT Toán lớp 7 Kết nối tri thức: Tìm x, biết:...

Tìm x, biết:

a)$- x + \dfrac{7}{4} = \dfrac{6}{5} - \dfrac{3}{4}$

b)$1 - 2x = \dfrac{9}{8} + \dfrac{7}{5}:\dfrac{2}{5}$

-Rút gọn vế phải

-Áp dụng quy tắc chuyển vế.

a)

$\begin{array}{l} - x + \dfrac{7}{4} = \dfrac{6}{5} - \dfrac{3}{4}\\ \Rightarrow  - x = \dfrac{6}{5} - \dfrac{3}{4} - \dfrac{7}{4}\\ \Rightarrow  - x = \dfrac{24}{20} - \dfrac{{15}}{20}-\dfrac{{35}}{20}\\ \Rightarrow  - x = \dfrac{{-26}}{{20}}\\ \Rightarrow x = \dfrac{{13}}{{10}}\end{array}$

Vậy $x=\dfrac{13}{10}$

b)

$\begin{array}{l}1 - 2x = \dfrac{9}{8} + \dfrac{7}{5}:\dfrac{2}{5}\\ \Rightarrow 1 - 2x = \dfrac{9}{8} + \dfrac{7}{5}.\dfrac{5}{2}\\\Rightarrow 1 - 2x = \dfrac{9}{8} + \dfrac{7}{2}\\ \Rightarrow 1 - 2x = \dfrac{{9 + 28}}{8}\\ \Rightarrow 2x = 1 - \dfrac{{37}}{8}\\ \Rightarrow 2x = \dfrac{{ - 29}}{8}\\ \Rightarrow x =  - \dfrac{{29}}{8}:2\\ \Rightarrow x =  - \dfrac{{29}}{8}.\dfrac{1}{2}\\ \Rightarrow x =  - \dfrac{{29}}{{16}}\end{array}$

Vậy $x=\dfrac{-29}{16}$