\(Bài1\\ 1,x^2-51=30\\ x^2=30+51\\ x^2=81\\ =>\left[{}\begin{matrix}x=9\\x=-9\end{matrix}\right.\\ -----------\\ 2,-3x^2+15=-4x^2+40\\ -3x^2+4x^2=40-15\\ x^2=25\\ =>\left[{}\begin{matrix}x=5\\x=-5\end{matrix}\right.\\ ----------\\ 3,-5x^2=-125\\ x^2=25\\ =>\left[{}\begin{matrix}x=5\\x=-5\end{matrix}\right.\\ ----------\\ 4,\left(5x+3\right)\left(x-3\right)-4x\left(x-3\right)=0\\ \left(x-3\right)\left(5x+3-4x\right)=0\\ \left(x-3\right)\left(x+3\right)=0\\ =>\left[{}\begin{matrix}x-3=0\\x+3=0\end{matrix}\right.=>\left[{}\begin{matrix}x=3\\x=-3\end{matrix}\right.\)
\(Bài2\\ 3\sqrt{5}=\sqrt{9.5}=\sqrt{45}\\ 7=\sqrt{49}\\ 45< 49\\ =>\sqrt{45}< \sqrt{49}\\ -->3\sqrt{5}< 7\)
