Giải bài 5 trang 15 SGK Toán 7 tập 1 – Chân trời sáng tạo

Đề bài

Tìm x, biết:

a)\(x.\frac{{14}}{{27}} = \frac{{ – 7}}{9}\)                

b)\(\left( {\frac{{ – 5}}{9}} \right):x = \frac{2}{3};\)

c)\(\frac{2}{5}:x = \frac{1}{{16}}:0,125\)      

d)\( – \frac{5}{{12}}x = \frac{2}{3} – \frac{1}{2}\)

Lời giải chi tiết

a)

\(\begin{array}{l}x.\frac{{14}}{{27}} = \frac{{ – 7}}{9}\\x = \frac{{ – 7}}{9}:\frac{{14}}{{27}}\\x = \frac{{ – 7}}{9}.\frac{{27}}{{14}}\\x = \frac{{ – 3}}{2}\end{array}\)                

Vậy \(x = \frac{{ – 3}}{2}\).

b)

\(\begin{array}{l}\left( {\frac{{ – 5}}{9}} \right):x = \frac{2}{3}\\x = \left( {\frac{{ – 5}}{9}} \right):\frac{2}{3}\\x = \left( {\frac{{ – 5}}{9}} \right).\frac{3}{2}\\x = \frac{{ – 5}}{6}\end{array}\)

Vậy \(x = \frac{{ – 5}}{6}\).

c)

\(\begin{array}{l}\frac{2}{5}:x = \frac{1}{{16}}:0,125\\\frac{2}{5}:x = \frac{1}{{16}}:\frac{1}{8}\\\frac{2}{5}:x = \frac{1}{{16}}.8\\\frac{2}{5}:x = \frac{1}{2}\\x = \frac{2}{5}:\frac{1}{2}\\x = \frac{2}{5}.2\\x = \frac{4}{5}\end{array}\)      

Vậy \(x = \frac{4}{5}\)

d)

\(\begin{array}{l} – \frac{5}{{12}}x = \frac{2}{3} – \frac{1}{2}\\ – \frac{5}{{12}}x = \frac{4}{6} – \frac{3}{6}\\ – \frac{5}{{12}}x = \frac{1}{6}\\x = \frac{1}{6}:\left( { – \frac{5}{{12}}} \right)\\x = \frac{1}{6}.\frac{{ – 12}}{5}\\x = \frac{{ – 2}}{5}\end{array}\)

Vậy \(x = \frac{{ – 2}}{5}\).

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