Bài 64 trang 34 SGK Toán 6 tập 2

Hoàn thành các phép tính:

a) \(\displaystyle {7 \over 9} - { \ldots  \over 3} = {1 \over 9}\)

b) \(\displaystyle {1 \over  \ldots } - {{ - 2} \over {15}} = {7 \over {15}}\)

c) \(\displaystyle {{ - 11} \over {14}} - {{ - 4} \over  \ldots } = {{ - 3} \over {14}}\)

d) \(\displaystyle { \ldots  \over {21}} - {2 \over 3} = {5 \over {21}}\) 

Thay dấu ba chấm bởi x rồi tìm x.

\(\displaystyle a)\;{7 \over 9} - {x \over 3} = {1 \over 9}\) . Từ đó suy ra: \(\displaystyle {x \over 3} = {7 \over 9} - {1 \over 9}={7-1 \over 9}\) hay \(\displaystyle {x \over 3} = {6 \over 9} = {2 \over 3}\)

Vậy x = 2 

b)

\(\dfrac{1}{x} - \dfrac{{ - 2}}{{15}} = \dfrac{7}{{15}} \)\(\Rightarrow \dfrac{1}{x} = \dfrac{7}{{15}} + \dfrac{{ - 2}}{{15}} = \dfrac{5}{{15}} = \dfrac{1}{3}\\
 \Rightarrow x = 3\)

c)

\(\dfrac{{ - 11}}{{14}} - \dfrac{{ - 4}}{x} = \dfrac{{ - 3}}{{14}} \)

\(\Rightarrow \dfrac{{ - 4}}{x} = \dfrac{{ - 11}}{{14}} - \dfrac{{ - 3}}{{14}} \)\(= \dfrac{{ - 11}}{{14}} + \dfrac{3}{{14}} = \dfrac{{ - 11 + 3}}{{14}} \)

\(= \dfrac{{ - 8}}{{14}} = \dfrac{{ - 4}}{7}\)

\( \Rightarrow x = 7\)

d)\(\dfrac{x}{{21}} - \dfrac{2}{3} = \dfrac{5}{{21}}\)\( \Rightarrow \dfrac{x}{{21}} = \dfrac{5}{{21}} + \dfrac{2}{3} \)\(= \dfrac{5}{{21}} + \dfrac{{14}}{{21}} = \dfrac{{19}}{{21}}\)

\( \Rightarrow x = 19\) 

Đáp số: a) x = 2 ; b) x = 3; c) x = 7; d) x =19.