Ta có : \(\left(27x^3+27x^2+9x\right)=26\)
\(\Leftrightarrow\left(27x^3+27x^2+9x+1\right)=27\)
\(\Leftrightarrow\left(3x+1\right)^3=27\) \(\Leftrightarrow3x+1=3\Leftrightarrow3x=2\Leftrightarrow x=\frac{2}{3}\)
Vậy ....
2x^3 + 27x^2 + 9x = 26
<=> 2x^3 + 27x^2 + 9x - 26 = 0
<=> (3x - 2)(9x^2 + 15x + 13) = 0
vì 9x^2 + 15x + 13 >= 0 nên:
<=> 3x - 2 = 0
<=> 3x = 2
<=> x = 2/3