\(\frac{1}{5}+\frac{1}{45}+\frac{1}{117}+\frac{1}{221}+...+\frac{1}{9021}+\frac{1}{9797}\)

\(=\frac{1}{1.5}+\frac{1}{5.9}+\frac{1}{9.13}+\frac{1}{13.17}+...+\frac{1}{93.97}+\frac{1}{97.101}\)

\(=\frac{1}{4}.\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+...+\frac{4}{93.97}+\frac{4}{97.101}\right)\)

\(=\frac{1}{4}.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+...+\frac{1}{97}-\frac{1}{101}\right)\)

\(=\frac{1}{4}.\left(1-\frac{1}{101}\right)\)

\(=\frac{1}{4}.\left(\frac{101}{101}-\frac{1}{101}\right)\)

\(=\frac{1}{4}.\frac{100}{101}\)

\(=\frac{25}{101}\)

1/5+1/45+1/117+1/221+...+1/9021+1/9797

=1/1.5+1/5.9+1/9.13+1/13.17+...+1/93.97+1/97.101

=1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+...+1/93-1/97+1/97-1/101

=1-1/101

=100/101

a, x = {45;60}

b, x = {35;42;49;56;63;70}

c, x = {36;54;72;90}

d, x = {36;48;60;72;84;96}

e, x = {25;30;35;40;45;50;55;60}

f, x = {27;30;33;36;39;42;45;48;51;54;57}

  hello

=240

k mk nha bn

thank  you

30 + 70 . 3

= 30 + 210

= 240

ai k tui, tui k lại

21-25-70+30+70 =

{70+30}-{70+25-21}=100-74

=36

123 x 30 + 123 x 70

= 123 x ( 30 + 70 )

= 123 x 100

= 12300

123 x 30 + 123 x 70

= 123 x ( 30 + 70 )

= 123 x 100

= 12300

a: \(x\in\left\{45;60\right\}\)

b: \(x\in\varnothing\)

thank you <3

a: Ta có: \(30⋮x\)

\(\Leftrightarrow x\in\left\{1;-1;2;-2;3;-3;5;-5;6;-6;10;-10;15;-15;30;-30\right\}\)

mà x<8

nên \(x\in\left\{1;2;3;5;6;-1;-2;-3;-5;-6;-10;-15;-30\right\}\)

b: Ta có: \(70⋮x\)

\(84⋮x\)

Do đó: \(x\inƯC\left(70;84\right)\)

\(\Leftrightarrow x\in\left\{1;2;7;14;-1;-2;-7;-14\right\}\)

mà x>8

nên x=14

c: Ta có: \(90⋮x\)

\(126⋮x\)

Do đó: \(x\inƯC\left(90;126\right)\)

\(\Leftrightarrow x\in\left\{1;-1;2;-2;3;-3;6;-6;9;-9;18;-18\right\}\)

mà x>9

nên x=18

d: Ta có: \(x⋮12\)

\(x⋮25\)

\(x⋮30\)

Do đó: \(x\in BC\left(12;25;30\right)\)

\(x\in\left\{...;-300;0;300;600;...\right\}\)

mà 0<x<500

nên x=300