Giải bài 124 trang 60 sách bài tập Toán 6 – Cánh Diều Tập 2

Đề bài

Tìm x, biết:

a) \( – 3x + 7 = 12 – 125\)

b) \(\frac{1}{3}:\left( {2x – 1} \right) = \frac{{ – 4}}{{21}}\)

c) \(\left[ {124 – \left( {20 – 4x} \right)} \right]:20 = 12\)

d) \(\left( {\frac{1}{{2.3}}\, + \,\frac{1}{{3.4}} + … + \frac{1}{{8.9}} + \,\frac{1}{{9.10}}} \right)\,.\,x = \frac{1}{5}\,\)

Lời giải chi tiết

a)

 \(\begin{array}{l} – 3x + 7 = 12 – 125\\ – 3x + 7 =  – 113\\ – 3x =  – 113 – 7\\ – 3x =  – 120\\x =  – 120:\left( { – 3} \right)\\x = 40\end{array}\)

b)

 \(\begin{array}{l}\frac{1}{3}:\left( {2x – 1} \right) = \frac{{ – 4}}{{21}}\\2x – 1 = \frac{1}{3}:\frac{{ – 4}}{{21}}\\2x – 1 = \frac{1}{3}.\frac{{21}}{{\left( { – 4} \right)}}\\2x – 1 = \frac{7}{{ – 4}}\\2x = 1 + \frac{{ – 7}}{4}\\2x = \frac{{ – 3}}{4}\\x = \frac{{ – 3}}{4}:2\\x = \frac{{ – 3}}{8}\end{array}\)

c)

 \(\begin{array}{l}\left[ {124 – \left( {20 – 4x} \right)} \right]:20 = 12\\124 – \left( {20 – 4x} \right) = 12.20\\124 – \left( {20 – 4x} \right) = 240\\20 – 4x = 124 – 240\\20 – 4x =  – 116\\4x = 20 – ( – 116)\\4x = 136\\x = 34\end{array}\)

d)

 \(\begin{array}{l}\left( {\frac{1}{{2.3}}\, + \,\frac{1}{{3.4}} + … + \frac{1}{{8.9}} + \,\frac{1}{{9.10}}} \right)\,.\,x = \frac{1}{5}\,\\\left( {\frac{1}{2}\, – \frac{1}{3} + \frac{1}{3} – \frac{1}{4} + … + \frac{1}{8} – \frac{1}{9} + \frac{1}{9} – \frac{1}{{10}}} \right)\,.\,x = \frac{1}{5}\,\\\left( {\frac{1}{2}\, – \frac{1}{{10}}} \right)\,.\,x = \frac{1}{5}\,\\\frac{4}{{10}}.x = \frac{1}{5}\\\frac{2}{5}.x = \frac{1}{5}\\x = \frac{1}{5}:\frac{2}{5}\\x = \frac{1}{2}\end{array}\)