\(\left\{{}\begin{matrix}3x+y=10\\x+y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x+y-x-y=10-4\\x+y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}2x=6\\x+y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}x=3\\y=4-x=4-3=1\end{matrix}\right.\)
\(x^4+3x^2-4=0\)
=>\(x^4+4x^2-x^2-4=0\)
=>\(\left(x^2+4\right)\left(x^2-1\right)=0\)
=>\(x^2-1=0\)
=>\(x^2=1\)
=>\(x\in\left\{1;-1\right\}\)
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