\(x^2\left(x-2\right)+x\left(-3+2x\right)-5x=7\)
\(x^3-2x^2-3x+2x^2-5x=7\)
\(x^3-8x-7=0\)
lm tiếp như sau nha bn!!
\(x^3-8x-7=0\)
\(\Leftrightarrow x^3-x^2+x^2-x-7x-7=0\)
\(\Leftrightarrow x^2\left(x-1\right)+x\left(x-1\right)-7\left(x+1\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(x^2+x\right)-7\left(x+1\right)=0\)
\(\Leftrightarrow x\left(x-1\right)\left(x+1\right)-7\left(x+1\right)=0\)
\(\Leftrightarrow\left(x+1\right)\left[x\left(x-1\right)-7\right]=0\)
\(\Leftrightarrow\left(x+1\right)\left(x^2-x-7\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x+1=0\\x^2-x-7=0\end{cases}\Leftrightarrow\orbr{\begin{cases}x=-1\\x^2-x-7=0\left(1\right)\end{cases}}}\)
Nếu bn hok sang kì II lp 9 thì (1) lm tiếp như sau:
\(x^2-x-7=0\)
\(\Delta=\left(-1\right)^2-4.1.\left(-7\right)=29>0\)
\(\Rightarrow x_1=\frac{1+\sqrt{29}}{2}\)
\(x_2=\frac{1-\sqrt{29}}{2}\)
=.= hok tốt!!
