1/2 x 1/3 + 1/3 x 1/4 + 1/4 x 1/5 + 1/5 x 1/6

= 1/3x( 1/2+1/4) + 1/5x(1/4+1/6)

= 1/4 + 1/12

= 1/3

5 giờ

     1 giờ 30 phút : 15  + 4 giờ 54 phút

=    90 phút : 15 + 4 giờ 54 phút

= 6 phút + 4 giờ 54 phút

= 4 giờ 60 phút

= 5 giờ 

\(\dfrac{2x-1}{2}+\dfrac{5-x}{6}=2-\dfrac{3\left(x+1\right)}{4}\)

\(\Leftrightarrow\dfrac{6\left(2x-1\right)}{12}+\dfrac{2\left(5-x\right)}{12}=\dfrac{24}{12}-\dfrac{9\left(x+1\right)}{12}\)

\(\Leftrightarrow6\left(2x-1\right)+2\left(5-x\right)=24-9\left(x+1\right)\)

\(\Leftrightarrow12x-6+10-2x=24-9x-9\)

\(\Leftrightarrow19x=11\)

`<=>x=11/19`

Vậy \(S=\left\{\dfrac{11}{19}\right\}\)

\(\dfrac{2x-1}{2}\) + \(\dfrac{5-x}{6}\) = 2 - \(\dfrac{3\left(x+1\right)}{4}\)

6(2x-1) + 2(5-x) = 24 - 9 (x+1)  (quy đồng mẫu số)

12x - 6 + 10 -2x = 24 - 9x - 9   (nhân phá ngoặc)

10x + 9x = 24 - 9  -10 + 6 ( chuyển vế đổi dấu)

19x = 11

x = 11/19

`(72+28)xx900=100 xx 900 = 90000`

khoảng cách giữa họ là 91 km sau

(91 - 67): 4 = 6 (giờ)

x: (2x -1) . (5x+3/4) =0  đk x # 1/2

⇔ x = 0 hoăc 5x + 3/4 = 0⇒ x = -3/4: 5 = -3/20

vậy x = {-3/20; 0}

`(2/3x - 1) . (5x + 3/4) = 0`

`=>` \(\left[{}\begin{matrix}\dfrac{2}{3}x-1=0\\5x+\dfrac{3}{4}=0\end{matrix}\right.\)

`=>` \(\left[{}\begin{matrix}\dfrac{2}{3}x=0+1\\5x=0-\dfrac{3}{4}\end{matrix}\right.\)

`=>` \(\left[{}\begin{matrix}\dfrac{2}{3}x=1\\5x=-\dfrac{3}{4}\end{matrix}\right.\)

`=>` \(\left[{}\begin{matrix}x=1:\dfrac{2}{3}\\x=-\dfrac{3}{4}:5\end{matrix}\right.\)

`=>` \(\left[{}\begin{matrix}x=\dfrac{3}{2}\\x=-\dfrac{3}{20}\end{matrix}\right.\)

Vậy......