Question

Tìm x biết

(x+2)³+(x-3)³

x³+31x²+155x+125=0

x³-4x²+12x-27=0

Answer 1

\(x^3+31x^2+155x+125=0\)

\(\Leftrightarrow x^3+3.x^2.5+3.x.5^2+125+16x^2+80x=0\)

\(\Leftrightarrow\left(x+5\right)^3+16x\left(x+5\right)=0\)

\(\Leftrightarrow\left(x+5\right).\left[\left(x+5\right)^2+16x\right]=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x+5=0\\\left(x+5\right)^2+16x=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-5\\x^2+26x+25=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-5\\x\left(x+26\right)=-25\left(2\right)\end{matrix}\right.\)

Đến đây bạn giải tiếp hệ (2) sẽ ra được x.