\(x\in\left\{0;1;2;3;4;5;6;7;8;9\right\}\)

\(0\le\frac{x}{5}\le2\Rightarrow\frac{x}{5}\in\left\{0;1;2\right\}\)

\(\Rightarrow x\in\left\{0;5;10\right\}\)

Gọi các ước nguyên tố của số N là p ; q ; r và p < q < r

\(\Rightarrow p=2;q+r=18\Rightarrow\orbr{\begin{cases}q=5;r=13\\q=7;r=11\end{cases}\Rightarrow\orbr{\begin{cases}N=2^a.5^b.13^c\\N=2^a.7^b.11^c\end{cases}}}\)

 Với a ; b; c \(\in\)N  và  \(\left(a+1\right)\left(b+1\right)\left(c+1\right)=12\Rightarrow12=2.2.3\)

Do đó N có thể là \(2^2.5.13;2.5^2.13;2.5.13^2;2^2.7.11;2.7^2.11;2.7.11^2\)

N nhỏ nhất nên \(N=2^2.5.13=260\)

\(A=\frac{1}{2}+\frac{1}{14}+\frac{1}{35}+\frac{1}{65}+\frac{1}{104}+\frac{1}{152}\)

\(A=\frac{1}{1.2}+\frac{1}{2.7}+\frac{1}{7.5}+\frac{1}{5.13}+\frac{1}{13.8}+\frac{1}{8.19}\)

\(A=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+\frac{2}{10.13}+\frac{2}{13.16}+\frac{2}{16.19}\)

\(A=\frac{2}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+\frac{3}{13.16}+\frac{3}{16.19}\right)\)

\(A=\frac{2}{3}.\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+\frac{1}{13}-\frac{1}{16}+\frac{1}{16}-\frac{1}{19}\right)\)

\(A=\frac{2}{3}.\left(1-\frac{1}{19}\right)\)

\(A=\frac{2}{3}.\frac{18}{19}\)

\(\Rightarrow A=\frac{12}{19}\)

\(A=\frac{1}{2}+\frac{1}{14}+\frac{1}{35}+\frac{1}{104}+\frac{1}{152}\)

\(A=\frac{2}{4}+\frac{2}{28}+\frac{2}{70}+\frac{2}{130}+\frac{2}{208}+\frac{2}{304}\)

\(A=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+\frac{2}{10.13}+\frac{2}{13.16}+\frac{2}{16.19}\)

\(A=\frac{2}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+\frac{3}{13.16}+\frac{3}{16.19}\right)\)

\(A=\frac{2}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+\frac{1}{13}-\frac{1}{16}+\frac{1}{16}-\frac{1}{19}\right)\)

\(A=\frac{2}{3}.\left(1-\frac{1}{19}\right)\)

\(A=\frac{2}{3}.\frac{18}{19}\)

\(A=\frac{12}{19}\)

\(x-\left(24-x\right)=x-19\)

\(x-24+x-x=19\)

\(x+x-x=24+19\)

\(x=43\)

x - ( 24 - x ) = x -19

=> x - 24 + x =  x - 19

=>  -24 = x - 19

=> -x = 24 - 19
=> -x = 5

=> x = -5

1: Tìm x

a,  x + 65 = 21

            x = 21 - 65

            x = -44

b, 12 - x = 30

           x = 12 - 30

           x = -18

2) Tính nhanh

SSH : (9-1):1+1= 9 ( số)

TBC:   (9+1):2 = 5]

Tổng là: 9 * 5 = 45

1, tìm x

a, x+ 65 = 67 - 56

=> x + 65 =  11

=> x= -54

vậy x= -54

b, 12 -x =5 x 6

=> 12 - x =30

=> x=-18

vậy x=-18

2, tính nhanh

1+2+3+4+5+6+7+8+9=(1+9)+(2+8)+(3+7)+(4+6)+5

=10 + 10+ 10+ 10+ 5=45

\(\frac{1}{2}\left(x-\frac{1}{3}\right)-3\left(x-\frac{1}{3}\right)=0\)

\(\Leftrightarrow\frac{1}{2}\left(x-\frac{1}{3}\right).2-3\left(x-\frac{1}{3}\right).2=0.2\)

\(\Leftrightarrow x-\frac{1}{3}-6\left(x-\frac{1}{3}\right)=0\)

\(\Leftrightarrow-5x+\frac{5}{3}=0\)

\(\Leftrightarrow-5x=0-\frac{5}{3}\)

\(\Leftrightarrow-5x=-\frac{5}{3}\)

\(\Leftrightarrow x=\left(-\frac{5}{3}\right):\left(-5\right)\)

\(\Leftrightarrow x=\frac{1}{3}\)

\(\Rightarrow x=\frac{1}{3}\)

1/2x(x-1/3) là 1/2 nhân (x-1/3) à