Question

1. Tìm x biết:

\(x^4-7x^2-8=0\)

Answer 1

\(x^4-7x^2-8=0\)

\(\Rightarrow x^4-8x^2+x^2-8=0\Rightarrow x^2\left(x^2-8\right)+x^2-8=0\)

\(\Rightarrow\left(x^2-8\right)\left(x^2+1\right)=0\)

\(\Rightarrow x^2-8=0\) (vì x² + 1 > 0)

\(\Rightarrow x^2=8\Rightarrow\orbr{\begin{cases}x=\sqrt{8}\\x=-\sqrt{8}\end{cases}}\)

Answer 2

x⁴-7x²-8

=x⁴+x²-8x²-8

=x²(x²+1)-8(x²+1)

=(x²-8)(x²+1)=0

TH1: x²-8=0  => x=...(loai)

TH2: x²+1=0  =>  x=1(Thoa man)

=>x=1

OK!!! Fighting supergirl!!!