Bài 1 :
a) 0,5x - \(\dfrac{2}{3}x=\dfrac{7}{12}\)
\(\dfrac{1}{2}x-\dfrac{2}{3}x=\dfrac{7}{12}\)
\(x.\left(\dfrac{1}{2}-\dfrac{2}{3}\right)\)= \(\dfrac{7}{12}\)
\(x.\dfrac{-1}{6}=\dfrac{7}{12}\)
\(x=\dfrac{7}{12}:\dfrac{-1}{6}\)
x = \(\dfrac{-7}{2}\).
b) \(x:4\dfrac{1}{3}=-2,5\)
\(x:\dfrac{13}{3}=\dfrac{-5}{2}\)
\(x=\dfrac{-5}{2}.\dfrac{13}{3}\)
\(x=\dfrac{-15}{26}\).
c) \(\left(\dfrac{3x}{7}+1\right):\left(-4\right)=\dfrac{-1}{28}\)
\(\dfrac{3x}{7}+1=\dfrac{-1}{28}.\left(-4\right)\)
\(\dfrac{3x}{7}+1=\dfrac{1}{7}\)
\(\dfrac{3x}{7}=\dfrac{1}{7}-1\)
\(\dfrac{3x}{7}=\dfrac{-6}{7}\)
=> 3x = -6
x = -6 :3 = -2.
Bài 2 : Bài giải :
Đổi \(26\dfrac{1}{4}\)km/h = 26,25 km/h .
Độ dài quãng đường AB là :
26,25 . 2,4 = 63 (km)
Thời gian người ấy đi từ B đến A là :
63 : 30 = 2,1 ( giờ) = 2 giờ 6 phút .
Bài 3 :
\(4\dfrac{3}{4}+\left(-0,37\right)+\dfrac{1}{8}+\left(-1,28\right)+\left(-2,5\right)+3\dfrac{1}{12}\)
= \(\dfrac{19}{4}+\dfrac{-37}{100}+\dfrac{1}{8}+\dfrac{-32}{25}+\dfrac{-5}{2}+\dfrac{37}{12}\)
= \(\left(\dfrac{19}{4}+\dfrac{1}{8}+\dfrac{-5}{2}\right)+\left(\dfrac{-37}{100}+\dfrac{-32}{25}\right)+\dfrac{37}{12}\)
= \(\dfrac{19}{8}+\dfrac{-33}{20}+\dfrac{37}{12}\)
= \(\dfrac{457}{120}\)
= \(3\dfrac{97}{120}\).
