Giải bài 60 trang 31 - SGK Toán lớp 7 Tập 1
Tìm \(x\) trong các tỉ lệ thức sau:
a) \(\left(\dfrac{1}{3}.x\right) : \dfrac{2}{3} = 1\dfrac{1}{4} : \dfrac{2}{5}\)
b) \(4,5 : 0,3 = 2,25 : (0,1.x)\)
c) \(8 : \left(\dfrac{1}{4}.x\right) = 2 : 0,02\)
d) \(3 : 2\dfrac{1}{4} = \dfrac{3}{4} : (6.x)\)
a) \(\left(\dfrac{1}{3}.x\right) : \dfrac{2}{3} = 1\dfrac{1}{4} : \dfrac{2}{5}\)
\(\Leftrightarrow \dfrac{1}{3}.x . \dfrac{3}{2} = \dfrac{5}{4} . \dfrac{5}{2}\)
\(\Leftrightarrow \dfrac{x}{2} = \dfrac{25}{8}\)
\(\Rightarrow x = \dfrac{25.2}{8} = \dfrac{25}{4}\)
Vậy \(x = \dfrac{25}{4}\)
b) \(4,5 : 0,3 = 2,25 : (0,1.x)\)
\(\Leftrightarrow 0,1.x = \dfrac{2,25.0,3}{4,5}\)
\(\Leftrightarrow 0,1.x = 0,15\)
\(\Rightarrow x = 0,15 : 0,1 = 1,5\)
Vậy \(x = 1,5\)
c) \(8 : \left(\dfrac{1}{4}.x\right) = 2 : 0,02\)
\(\Leftrightarrow 8.\dfrac{4}{x} = 100\)
\(\Leftrightarrow \dfrac{32}{x} = 100\)
\(\Rightarrow x = \dfrac{32}{100} = 0,32\)
Vậy \(x = 0,32\)
d) \(3 : 2\dfrac{1}{4} = \dfrac{3}{4} : (6.x)\)
\(\Leftrightarrow 3:\dfrac{9}{4} = \dfrac{3}{24x} \)
\(\Leftrightarrow \dfrac{12}{9} = \dfrac{3}{24x} \)
\(\Leftrightarrow \dfrac{1}{8x} = \dfrac{4}{3} \)
\(\Rightarrow x = \dfrac{3}{4.8} = \dfrac{3}{32}\)
Vậy \(x = \dfrac{3}{32} \)
Lưu ý:
Nếu \(\dfrac{a}{b} = \dfrac{c}{d}\) thì \(a.d = b.c.\)
