Question

[nguyen minh ngoc]

Help me!!

Answer 1

[Nguyễn Huy Tú]

Bài 11:

a, Đặt \(A=x-x^2=-\left(x^2+x\right)=-\left(x^2-\dfrac{1}{2}.x.2+\dfrac{1}{4}-\dfrac{1}{4}\right)\)

\(=-\left[\left(x-\dfrac{1}{2}\right)^2-\dfrac{1}{4}\right]=-\left(x-\dfrac{1}{2}\right)^2+\dfrac{1}{4}\)

Ta có: \(A=-\left(x-\dfrac{1}{2}\right)^2+\dfrac{1}{4}\le\dfrac{1}{4}\)

Dấu " = " khi \(-\left(x-\dfrac{1}{2}\right)^2=0\Leftrightarrow x=\dfrac{1}{2}\)

Vậy \(MAX_A=\dfrac{1}{4}\) khi \(x=\dfrac{1}{2}\)

b, Đặt \(B=4x-x^2+3=-\left(x^2-4x-3\right)\)

\(=-\left(x^2-4x+4-7\right)\)

\(=-\left[\left(x-2\right)^2-7\right]\)

\(=-\left(x-2\right)^2+7\le7\)

Dấu " = " khi \(-\left(x-2\right)^2=0\Leftrightarrow x=2\)

Vậy \(MAX_B=7\) khi x = 2

Answer 2

[T.Thùy Ninh]

\(a,A=x^2-20x+101=\left(x^2-20x+100\right)+1=\left(x-10\right)^2+1\ge1\)Vậy \(Min_A=1\) khi \(x-10=0\Rightarrow x=10\)

\(B=4x^2+4x+2=4\left(x^2+2x+1\right)-2=4\left(x+1\right)^2-2\ge-2\)Vậy \(Min_B=-2\) khi \(x+1=0\Rightarrow x=-1\)

\(c,x^2-4xy+5y^2+10x-22y+28=\left(x^2-4xy+10x\right)+5y^2-22y+28\)\(=\left[x^2-2x\left(2y-5\right)+\left(2y-5\right)^2\right]+5y^2-22y+28-4y^2+20x-25\)\(=\left[x-\left(2y-5\right)\right]^2+\left(y-2x+1\right)+2\)

\(=\left(x-2y+5\right)^2+\left(y-1\right)^2+2\ge2\)

Vậy \(Min_C=2\) khi \(\left[{}\begin{matrix}x-2y+5=0\\y-1=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x-2+5=0\\y=1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x+3=0\\y=1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=-3\\y=1\end{matrix}\right.\)Bài 11:

\(a,x-x^2=-\left(x^2-x+\dfrac{1}{4}\right)+\dfrac{1}{4}=-\left(x-\dfrac{1}{2}\right)^2+\dfrac{1}{4}\le\dfrac{1}{4}\)Vậy GTLN của biểu thức là \(\dfrac{1}{4}\) khi \(x-\dfrac{1}{2}=0\Rightarrow x=\dfrac{1}{2}\)

\(b,4x-x^2+3=7-\left(4-4x+x^2\right)=7-\left(2-x\right)^2\le7\)Vậy \(\) GTLN của biểu thức là 7 khi \(2-x=0\Rightarrow x=2\)