a/ \(m=2\\→x^2-4x-1=0\\Δ=(-4)^2-4.1.(-1)=20>0\\→Pt\,\,co\,\,2\,\,nghiem\,\,phan\,\,biet:\\x_1=4+\sqrt{20}\\x_2=4-\sqrt{20}\)
b/ \(Δ=(-2m)^2-4.1.(-1)=4m^2+4>0∀m\\→Pt\,\,co\,\,2\,\,nghiem\,\,phan\,\,biet\,\,∀m\)
c/ \(Theo\,\, Vi-et:\\\begin{cases}x_1+x_2=2m\\x_1x_2=-1\end{cases}\\x_1^2+x_2^2-x_1x_2=7\\↔(x_1+x_2)^2-3x_1x_2=7\\↔(2m)^2-3.(-1)=0\\↔4m^2+3=0(vo\,\,ly)\\→m∈\varnothing\)
