ta có : 

\(\frac{x+1}{3}+\frac{x+1}{9}+\frac{x+1}{27}+\frac{x+1}{81}=\left(x+1\right)\times\left(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}\right)\)

\(=\left(x+1\right)\times\frac{40}{81}=\frac{56}{81}\text{ nên }x+1=\frac{56}{40}=\frac{7}{5}\)

vậy \(x=\frac{7}{5}-1=\frac{2}{5}\)

16/81

81:x=9

     x=81:9

 Vậy x=9

\(81:x=9\)

        \(x=81:9\)

        \(x=9\)

Vậy  \(x=9\)

Đáp án D

a, -1

b, -9

Chọn A

9. x = − 81 ⇔ x = − 81 : 9 ⇔ x = − 9

\(\left(x+\dfrac{2}{9}\right)^2=\dfrac{4}{81}\\ \left(x+\dfrac{2}{9}\right)^2=\left(\dfrac{2}{9}\right)^2\\ x+\dfrac{2}{9}=\dfrac{2}{9}\\ x=\dfrac{2}{9}-\dfrac{2}{9}\\ x=0\)

Mình bổ sung thêm TH2 nữa. 

\(\left(x+\dfrac{2}{9}\right)^2=\left(-\dfrac{2}{9}\right)^2\\ x+\dfrac{2}{9}=-\dfrac{2}{9}\\ x=-\dfrac{2}{9}-\dfrac{2}{9}\\ x=-\dfrac{4}{9}\)

\(...\Rightarrow\left(x+\dfrac{2}{9}\right)^2=\left(\dfrac{2}{9}\right)^2\)

\(\Rightarrow x+\dfrac{2}{9}=\dfrac{2}{9}\Rightarrow x=0\)

\(x\): 9 = 81

        \(x\)= 81 x 9

         \(x\)= 729

X:9=81

    X=81X9

    X=729

x:9=81

x= 81*9

x= 729