Bạn ghi đề lại câu a.

(1+2+3+4+5+6+7+8+9)xX=405

Số các số hạng:

(9-1):1+1=9 số

Tổng các số hạng:

(1+9)x9:2=45

X=405:45

X=9.

\(\left(x.1\right)+\left(x.2\right)+\left(x.3\right)+...+\left(x.9\right)=405\)

\(x.\left(1+2+3+...+9\right)=405\)

\(x.45=405\)

\(x=405\div45\)

\(x=9\)

6x = 24

  x = 24 : 6

  x = 4

Vậy x = 4

Tính M=4÷(7×31)+6÷(7×41)+9÷(10×41)+7÷(10×57)

\(3^{x+2}+3^{x+1}+3^4=405\)

\(3^{x+1}.3+3^{x+1}+81=405\)

\(3^{x+1}.\left(3+1\right)=405-81\)

\(3^{x+1}.4=324\)

\(3^{x+1}=81\)

\(3^{x+1}=3^4\)

\(x+1=4\)

\(x=3\)

nhanh lên các bạn nhé !

\(4\cdot\left(x-12\right)=2x+164\)

\(\Leftrightarrow4x-48=2x+164\)

\(\Leftrightarrow4x-2x=164+48\)

\(\Leftrightarrow2x=212\Rightarrow x=106\)

Sửa đề : \(\frac{1}{1.5}+\frac{1}{5.9}+...+\frac{1}{401.405}+x=2\frac{1}{405}\)

\(\Leftrightarrow\frac{1}{4}\cdot\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+...+\frac{1}{401}-\frac{1}{405}\right)+x=\frac{811}{405}\)

\(\Leftrightarrow\frac{1}{4}\cdot\left(1-\frac{1}{405}\right)+x=\frac{811}{405}\)

\(\Leftrightarrow\frac{1}{4}\cdot\frac{404}{405}+x=\frac{811}{405}\)

\(\Leftrightarrow\frac{101}{405}+x=\frac{811}{405}\Rightarrow x=\frac{142}{81}\)

   4x(X-12)=2xX+164

2x2x(X-12)=2xX+2x82

  2x(2X-24)=2x(X+82)

        2X-24=X+82

            2X=X+82+24

            2X=X+106

             X=106

\(\left(x\times1\right)+\left(x\times2\right)+...+\left(x\times9\right)=405\)

\(\Rightarrow x\left(1+2+3+...+9\right)=405\)

\(\Rightarrow x\times45=405\)

\(\Rightarrow x=405:45\)

\(\Rightarrow x=9\)

Xyz sao mình tính được là 405:50 là có dư

( X x 1 ) + ( X x 2 ) + ( X x 3 ) +.....+ ( X x 9 ) = 405

X x ( 1 + 2 +3 + .....+ 9 ) = 405

X x 45 = 405

    X     = 405: 45 

    X     =  9

\(\dfrac{2}{3}.3^{x+1}-7.3^x=-405\)

\(\Rightarrow\dfrac{2}{3}.3^x.3^1-7.3^x=-405\)

\(\Rightarrow2.3^x-7.3^x=-405\)

\(\Rightarrow3^x.\left(2-7\right)=-405\)

\(\Rightarrow3^x.-5=-405\)

\(\Rightarrow3^x=\dfrac{-405}{-5}\)

\(\Rightarrow3^x=81\)

Vì \(3^4=81\) nên \(x=4\)

\(\dfrac{2}{3}.3^{x+1}-7.3^x=-405\)

\(\Leftrightarrow\dfrac{2}{3}.3.3^x-7.3^x=-405\)

\(\Leftrightarrow3^x\left(2-7\right)=-405\)

\(\Leftrightarrow3^x.\left(-5\right)=-405\)

\(\Leftrightarrow3^x=81\)

\(\Leftrightarrow3^x=3^4\)

\(\Leftrightarrow x=4\)

Vậy..

x- 1/5=2x+1/3

x-2x=1/5+1/3

-x= 8/15

=> x= -8/15

bn ơi đề câu 2 bn viết x+3/2 rồi nhân hay x đó

b: \(\dfrac{x+3}{2x-5}=\dfrac{2}{3}\)

=>4x-10=3x+9

=>x=19

c: \(\Leftrightarrow3^x\cdot\left(\dfrac{2}{3}\cdot3-7\right)=-405\)

=>3^x=81

=>x=4

d: \(\Leftrightarrow2^x\cdot40+2^x\cdot12+2^x\cdot48=200\)

=>2^x=2

=>x=1