a) \(N=13-6x-3x^2\)
\(-N=3x^2+6x-13\)
\(-N=3\left(x^2+2x+1\right)-16\)
\(-N=3\left(x+1\right)^2-16\)
Mà \(\left(x+1\right)^2\ge0\forall x\)\(\Rightarrow3\left(x+1\right)^2\ge0\forall x\)
\(\Rightarrow-N\ge-16\)
\(\Leftrightarrow N\le16\)
Dấu "=" xảy ra khi : \(x+1=0\Leftrightarrow x=-1\)
Vậy ...
b) \(Q=2x^2-8x+11\)
\(Q=2\left(x^2-4x+4\right)+3\)
\(Q=2\left(x-2\right)^2+3\)
Mà \(\left(x-2\right)^2\ge0\forall x\) \(\Rightarrow2\left(x-2\right)^2\ge0\forall x\)
\(\Rightarrow Q\ge3\)
Dấu "=" xảy ra khi : \(x-2=0\Leftrightarrow x=2\)
Vậy ...
ran love shinichi................
CẠN CMN LỜI
