Toán Lớp 6: 1.Tìm x
a, 4/9 + 5/6 : x = 1/6
b, 5/4 : x + 2 = 2/3
c, (1/7 – 1/3) x = 28/3 . (1/4 – 1/7)
d, 13/15 – (13/21 + x) . 7/12 = 7/10
2. Tính
a, (15/21 : 5) : (6/5 : 2/3)
b, (8/21 . 7) : (1/3 . 6/11)
c, C = 2/3 + 2/5 – 2/9 phần 3/13 + 3/25 – 3/45
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Comments ( 2 )
1)a)\dfrac{4}{9} + \dfrac{5}{6}:x = \dfrac{1}{6}\\
\Leftrightarrow \dfrac{5}{6}:x = \dfrac{1}{6} – \dfrac{4}{9}\\
\Leftrightarrow \dfrac{5}{6}:x = \dfrac{{ – 5}}{{18}}\\
\Leftrightarrow x = \dfrac{5}{6}:\dfrac{{ – 5}}{{18}}\\
\Leftrightarrow x = \dfrac{5}{6}.\dfrac{{ – 18}}{5}\\
\Leftrightarrow x = – 3\\
Vậy\,x = – 3\\
b)\dfrac{5}{4}:x + 2 = \dfrac{2}{3}\\
\Leftrightarrow \dfrac{5}{4}:x = \dfrac{2}{3} – 2\\
\Leftrightarrow \dfrac{5}{4}:x = \dfrac{{ – 4}}{3}\\
\Leftrightarrow x = \dfrac{5}{4}:\dfrac{{ – 4}}{3} = \dfrac{5}{4}.\dfrac{{ – 3}}{4}\\
\Leftrightarrow x = \dfrac{{ – 15}}{{16}}\\
Vậy\,x = \dfrac{{ – 15}}{{16}}\\
c)\left( {\dfrac{1}{7} – \dfrac{1}{3}} \right).x = \dfrac{{28}}{3}.\left( {\dfrac{1}{4} – \dfrac{1}{7}} \right)\\
\Leftrightarrow \dfrac{{ – 4}}{{21}}.x = \dfrac{{28}}{3}.\dfrac{3}{{28}}\\
\Leftrightarrow \dfrac{{ – 4}}{{21}}.x = 1\\
\Leftrightarrow x = \dfrac{{ – 21}}{4}\\
Vậy\,x = \dfrac{{ – 21}}{4}\\
2)\\
a)\left( {\dfrac{{15}}{{21}}:5} \right):\left( {\dfrac{6}{5}:\dfrac{2}{3}} \right)\\
= \dfrac{3}{{21}}:\left( {\dfrac{6}{5}.\dfrac{3}{2}} \right)\\
= \dfrac{1}{7}:\dfrac{9}{5}\\
= \dfrac{1}{7}.\dfrac{5}{9}\\
= \dfrac{5}{{63}}\\
b)\left( {\dfrac{8}{{21}}.7} \right):\left( {\dfrac{1}{3}.\dfrac{6}{{11}}} \right)\\
= \dfrac{8}{3}:\dfrac{2}{{11}}\\
= \dfrac{8}{3}.\dfrac{{11}}{2}\\
= \dfrac{{44}}{3}\\
c)C = \dfrac{{\dfrac{2}{3} + \dfrac{2}{5} – \dfrac{2}{9}}}{{\dfrac{3}{{15}} + \dfrac{3}{{25}} – \dfrac{3}{{45}}}}\\
= \dfrac{{2.\left( {\dfrac{1}{3} + \dfrac{1}{5} – \dfrac{1}{9}} \right)}}{{\dfrac{3}{5}.\left( {\dfrac{1}{3} + \dfrac{1}{5} – \dfrac{1}{9}} \right)}}\\
= \dfrac{2}{{\dfrac{3}{5}}} = \dfrac{{10}}{3}
\end{array}$