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Tìm số x trong mỗi tỉ lệ thức sau:
a) \(\dfrac{x}{{26}} = \dfrac{{21}}{{39}}\); b) \(4:5,2 = x:\dfrac{2}{7}\);
c) \(\dfrac{{1,25}}{{0,1x}} = \dfrac{{1,35}}{{0,2}}\); d*) \((3x – 2):\dfrac{7}{2} = \dfrac{4}{{21}}:\dfrac{1}{{12}}\).
Tìm x dựa vào các tỉ lệ thức đã cho.
Nếu \(\dfrac{a}{b} = \dfrac{c}{d} \Rightarrow ad = bc\).
a)
\(\begin{array}{l}\dfrac{x}{{26}} = \dfrac{{21}}{{39}}\\{\rm{ }}39.x = 21.26\\{\rm{ }}39.x = 546\\{\rm{ }}x = 546:39\\{\rm{ }}x = 14\end{array}\)
Advertisements (Quảng cáo)
Vậy \(x = 14\).
b)
\(\begin{array}{l}4:5,2 = x:\dfrac{2}{7}\\{\rm{ }}4:\dfrac{{26}}{5} = x:\dfrac{2}{7}\\{\rm{ }}x:\dfrac{2}{7} = 4.\dfrac{5}{{26}}\\{\rm{ }}x:\dfrac{2}{7} = \dfrac{{10}}{{13}}\\{\rm{ }}x = \dfrac{{10}}{{13}}.\dfrac{2}{7}\\{\rm{ }}x = \dfrac{{20}}{{91}}\end{array}\)
Vậy \(x = \dfrac{{20}}{{91}}\).
c)
\(\begin{array}{l}\dfrac{{1,25}}{{0,1x}} = \dfrac{{1,35}}{{0,2}}\\{\rm{ }}1,25{\rm{ }}.{\rm{ }}0,2 = 1,35{\rm{ }}.{\rm{ }}0,1x\\{\rm{ }}0,25 = 0,135x\\{\rm{ }}x = 0,25:0,135\\{\rm{ }}x = \dfrac{{50}}{{27}}\end{array}\)
Vậy \({\rm{ }}x = \dfrac{{50}}{{27}}\).
d*)
\(\begin{array}{l}(3x – 2):\dfrac{7}{2} = \dfrac{4}{{21}}:\dfrac{1}{{12}}\\{\rm{ }}(3x – 2).\dfrac{2}{7} = \dfrac{4}{{21}}.12\\{\rm{ }}\dfrac{{2.(3x – 2)}}{7} = \dfrac{{16}}{7}\\ \to 2.(3x – 2) = 16\\{\rm{ }}3x – 2 = 16:2 = 8\\{\rm{ }}3x = 8 + 2 = 10\\{\rm{ }}x = 10:3\\{\rm{ }}x = \dfrac{{10}}{3}\end{array}\)
Vậy \(x = \dfrac{{10}}{3}\).