a. \(3x^2-12x+9\)
⇔Δ=b'\(^2\) - 4ac= (-6)\(^2\) - 3.9=36 - 27=11>0
\(\sqrt{\Delta}=\sqrt{11}\)
vây pt có 2 nghiệm phân biệt
\(x_1=\dfrac{-b'+\sqrt{\Delta}'}{a}\)=\(\dfrac{6+\sqrt{11}}{3}\)
\(x_2=\dfrac{-b'-\sqrt{\Delta}'}{a}=\)\(\dfrac{6-\sqrt{11}}{3}\)
b.-2021x\(^2\)+2020x+1=0
thấy có: a + b +c =-2021 + 2020 +1=0 nên pt có nghiệm x\(_1\)=1
x\(_2\)=\(\dfrac{c}{a}=\dfrac{1}{-2021}\)
c.x\(^2\)-3x + 1=0
Δ=b\(^2\)-4ac= (-3)\(^2\)- 4.1.1= 9-4=5>0
\(\sqrt{\Delta}=\sqrt{5}\)
vậy pt có 2 nghiệm phân biệt
\(x_1=\dfrac{-b+\sqrt{\Delta}}{2a}=\)\(\dfrac{3+\sqrt{5}}{2}\)
\(x_2=\dfrac{-b-\sqrt{\Delta}}{2a}=\)\(\dfrac{3-\sqrt{5}}{2}\)
