Question

Cần gấp ạ,chi tiết nhé

Answer 1

 

Câu 8:

a: \(7x^2-4x-3< 0\)

=>(x-1)(7x+3)<0

=>\(-\dfrac{3}{7}< x< 1\)

=>sai

b: \(-x^2+6x-9\)

\(=-\left(x^2-6x+9\right)=-\left(x-3\right)^2< =0\forall x\)

mà \(-x^2+6x-9>=0\)

nên x-3=0

=>x=3

=>Sai

c: \(-5x^2+4x+12< 0\)

=>\(5x^2-4x-12>0\)

=>\(5x^2-10x+6x-12>0\)

=>(x-2)(5x+6)>0

=>\(\left[{}\begin{matrix}x>2\\x< -\dfrac{6}{5}\end{matrix}\right.\)

=>Đúng

d: \(3x^2-4x+4=3\left(x^2-\dfrac{4}{3}x+\dfrac{4}{3}\right)\)

\(=3\left(x^2-2\cdot x\cdot\dfrac{2}{3}+\dfrac{4}{9}+\dfrac{8}{9}\right)\)

\(=3\left(x-\dfrac{2}{3}\right)^2+\dfrac{8}{3}>=\dfrac{8}{3}>0\forall x\)

=>Đúng