A=-(x^2-4x-2) =-(x-2)^2+6 =<6
Max A=6 khi x=2
B=-(x^2 -x-2)= -(x-1/2)^2+9/4=<9/4
Max B=9/4 khi x=1/2
\(A=-\left(x^2-4x-2\right)=-\left(x^2-4x+4-6\right)=-\left(x-2\right)^2+6\le6\)
dấu"=" xảy ra<=>x=2
\(B=-\left(x^2-x-2\right)=-\left(x^2-2.\dfrac{1}{2}x+\dfrac{1}{4}-\dfrac{9}{4}\right)\)
\(=-\left(x-\dfrac{1}{2}\right)^2+\dfrac{9}{4}\le\dfrac{9}{4}\) dấu""=" xảy ra<=>x=1/2
a) Ta có: \(A=-x^2+4x+2\)
\(=-\left(x^2-4x-2\right)\)
\(=-\left(x-2\right)^2+6\le6\forall x\)
Dấu '=' xảy ra khi x=2
b) Ta có: \(B=-x^2+x+2\)
\(=-\left(x^2-2\cdot x\cdot\dfrac{1}{2}+\dfrac{1}{4}-\dfrac{9}{4}\right)\)
\(=-\left(x-\dfrac{1}{2}\right)^2+\dfrac{9}{4}\le\dfrac{9}{4}\forall x\)
Dấu '=' xảy ra khi \(x=\dfrac{1}{2}\)
