\(12=2^2.3\\ 21=3.7\\ \Rightarrow BCNN\left(12,21\right)=2^2.3.7=84\\ \Rightarrow x\in BC\left(12,21\right)=B\left(84\right)=\left\{0;84;168;252;...\right\}\\ \text{Mà }100< x< 200\Rightarrow x=168\)

\(x\in BC\left(12;21\right)\)

mà 100<x<200

nên x=168

\(a,\left(x-5\right).\frac{30}{100}=\frac{200x}{100}+5\)

\(\left(x-5\right).\frac{3}{10}=2x+5\)

\(\frac{3x}{10}-\frac{3}{2}=2x+5\)

\(\frac{3}{10}x-2x=\frac{3}{2}+5\)

\(x\left(\frac{3}{10}-2\right)=\frac{13}{2}\)

\(x.\frac{-17}{10}=\frac{13}{2}\)

\(x=\frac{13}{2}:\frac{-17}{10}\)

\(x=\frac{13}{2}\cdot\frac{-10}{17}\)

\(x=\frac{-65}{17}\)

Vậy \(x=\frac{-65}{17}\)

Bài 2:

Ta có:\(\frac{-5x}{21}+\frac{-5y}{21}+\frac{-5z}{21}=\frac{-5}{21}\left(x+y+z\right)\)

Mà đề bài cho \(x+y=-z\Rightarrow\frac{-5}{21}\left(-z+z\right)=\frac{-5}{21}.0=0\Rightarrow A=0\)

Vậy \(A=0\)

a: =197+3*(34-64)

=197-3*30

=197-90=107

b: =25(134-34)+100=100*26=2600

d: =>x thuộc B(84)

mà 100<x<200

nên x=168

• 20+21+22+...+100         

           Số số hạng của dãy là:

                  [100-20]:1+1=81

           Tổng của dãy là:

                  [100+20].81:2=4860 

a)=9600

X+100+100+200=200

X+(100+100+200)=200

X+400=200

X=200-400

X=-200

x + 100 + 100 + 200 = 200

x                             = 200 - 200 - 100 - 100

x                             = - 200

\(x:100=200+100\)

\(x:100=300\)

          \(x=300x100\)

         \(x=30000\)

x : 100 = 200 + 100

x : 100 = 300

x          = 300 x 100

x           = 30000

x+200+200=200+200+100

x+400=500

x=500-400

x=100

tk minh nha

X+200+200=200+200+100

X+(200+200)=500

X+400=500

X=500-400

X=100

x + 100 = 200 - 400 + 100

x + 100 = -100

x = -100 - 100

x = -200

x+100=200-400+100

x+100=-200+100

x+100=-100

x=-100-100

x=-200

x + 100 = 200 - 300 + 100

x + 100 = -100 + 100

x + 100 = 0

x = 0 - 100

x = -100

x+100=200-300+100

x+100=-100+100

x+100=0

x=0-100

x=-100

k mk nhé

x + 100 = 200 - 100 - 100

x + 100 = 0

x          = 0 - 100

x          = -100

Vậy x = - 100

x+100=200-100-100

x+100=0

x       =0-100

x       =-100