Tìm số hữu tỉ x, biết:
a) \(\dfrac{3}{7} + x = – 1,5\);
b) \(3\dfrac{1}{5} – x = 1,6 + \dfrac{7}{{10}}\);
c) \(x.\dfrac{{14}}{3} = 2,5\);
d) \(x:\left( { – \dfrac{3}{5}} \right) = 1\dfrac{1}{4}\).
Tìm x dựa vào các phép tính đã cho. (Thực hiện các bước cộng, trừ, nhân, chia để tính x).
a)
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\(\begin{array}{l}\dfrac{3}{7} + x = – 1,5\\{\rm{ }}\dfrac{3}{7} + x = \dfrac{{ – 3}}{2}\\{\rm{ }}x = \dfrac{{ – 3}}{2} – \dfrac{3}{7}\\{\rm{ }}x = \dfrac{{ – 21}}{{14}} – \dfrac{6}{{14}}\\{\rm{ }}x = \dfrac{{ – 27}}{{14}}\end{array}\)
Vậy \(x = \dfrac{{ – 27}}{{14}}\).
b)
\(\begin{array}{l}3\dfrac{1}{5} – x = 1,6 + \dfrac{7}{{10}}\\{\rm{ }}\dfrac{{16}}{5} – x = \dfrac{{16}}{{10}} + \dfrac{7}{{10}}\\{\rm{ }}\dfrac{{16}}{5} – x = \dfrac{{23}}{{10}}\\{\rm{ }}x = \dfrac{{16}}{5} – \dfrac{{23}}{{10}}\\{\rm{ }}x = \dfrac{{32}}{{10}} – \dfrac{{23}}{{10}}\\{\rm{ }}x = \dfrac{9}{{10}}\end{array}\)
Vậy \(x = \dfrac{9}{{10}}\).
c)
\(\begin{array}{l}x.\dfrac{{14}}{3} = 2,5\\{\rm{ }}x.\dfrac{{14}}{3} = \dfrac{5}{2}\\{\rm{ }}x = \dfrac{5}{2}:\dfrac{{14}}{3}\\{\rm{ }}x = \dfrac{5}{2}.\dfrac{3}{{14}}\\{\rm{ }}x = \dfrac{{15}}{{28}}\end{array}\)
Vậy \(x = \dfrac{{15}}{{28}}\).
d)
\(\begin{array}{l}x:\left( { – \dfrac{3}{5}} \right) = 1\dfrac{1}{4}\\{\rm{ }}x:\left( { – \dfrac{3}{5}} \right) = \dfrac{5}{4}\\{\rm{ }}x = \dfrac{5}{4}.\left( { – \dfrac{3}{5}} \right)\\{\rm{ }}x = – \dfrac{3}{4}\end{array}\)
Vậy \(x = – \dfrac{3}{4}\).