Tìm giá trị lớn nhất của biểu thức :
P = x^2 +y^2 + 3
x^2 + y^2 + 2
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1 )Vì \(\left(x+2\right)^2\ge0;\left(y-3\right)^2\ge0\)
\(\Rightarrow\left(x+2\right)^2+\left(y-3\right)^2\ge0\)
\(\Rightarrow\left(x+2\right)^2+\left(y-3\right)^2+1\ge1\)
Dấu "=: xảy ra <=> \(\orbr{\begin{cases}\left(x+2\right)^2=0\\\left(y-3\right)^2=0\end{cases}\Rightarrow\orbr{\begin{cases}x=-2\\y=3\end{cases}}}\)
Vậy ........
2 ) \(\frac{1}{\left(x-2\right)^2+2}\ge\frac{1}{2}\)
Dấu "=" xảy ra <=> x = 2
Vậy ..........
có: \(\dfrac{1}{x^2+y^2}=\dfrac{1}{\left(x+y\right)^2-2xy}=\dfrac{1}{1-2xy}\)(1)
có \(\dfrac{1}{xy}=\dfrac{2}{2xy}\left(2\right)\)
từ(1)(2)=>A=\(\dfrac{1}{1-2xy}+\dfrac{2}{2xy}\ge\dfrac{\left(1+\sqrt{2}\right)^2}{1}=\left(1+\sqrt{2}\right)^2\)
=>Min A=(1+\(\sqrt{2}\))^2
Từ gt ta có x^2+y^^2=xy+1
=>P=(x^2+y^2)^2-2x^2y^2-x^2y^2
=(xy+1)2-2x2y2-x2y2
=x2y2+xy+1-3x2y2=-2x2y2+xy+1
=......
\(1=x^2+y^2-xy\ge2xy-xy=xy\Rightarrow xy\le1\)
\(1=x^2+y^2-xy\ge-2xy-xy=-3xy\Rightarrow xy\ge-\dfrac{1}{3}\)
\(\Rightarrow-\dfrac{1}{3}\le xy\le1\)
\(P=\left(x^2+y^2\right)^2-2\left(xy\right)^2-\left(xy\right)^2=\left(xy+1\right)^2-3\left(xy\right)^2=-2\left(xy\right)^2+2xy+1\)
Đặt \(xy=t\in\left[-\dfrac{1}{3};1\right]\)
\(P=f\left(t\right)=-2t^2+2t+1\)
\(f'\left(t\right)=-4t+2=0\Rightarrow t=\dfrac{1}{2}\)
\(f\left(-\dfrac{1}{3}\right)=\dfrac{1}{9}\) ; \(f\left(\dfrac{1}{2}\right)=\dfrac{3}{2}\) ; \(f\left(1\right)=1\)
\(\Rightarrow P_{max}=\dfrac{3}{2}\) ; \(P_{min}=\dfrac{1}{9}\)
Vì | x - 3 | \(\ge\)0 ( 1 )
=> | x - 3 | + 2 \(\ge\)2
=> ( | x - 3 | + 2 )2 \(\ge\) 22 = 4
Vì | y + 3 | \(\ge\) 0 ( 2 )
Từ ( 1 ) và ( 2 ) => ( | x - 3 | + 2 )2 + | y + 3 | + 2007 \(\ge\) 4 + 0 + 2007
=> P \(\ge\) 2011
Dấu "=" xảy ra khi | x - 3 | = 0 và | y + 3 | = 0
=> x - 3 = 0 và y + 3 = 0
=> x = 3 và y = -3
Vậy GTNN của P là 2011 khi ( x ; y ) = ( 3 ; -3 )
\(A=\frac{3}{\left(x+2\right)^2+4};\left(x+2\right)^2\in N\)
\(\Rightarrow A_{max}\Leftrightarrow\left(x+2\right)^2=0\Leftrightarrow\left(x+2\right)^2+4=4\)
\(\Rightarrow A_{max}=\frac{3}{4}\)
b, \(B=\left(x+1\right)^2+\left(y+3\right)^2+1\)
Mặt khác: \(\left(x+1\right)^2;\left(y+3\right)^2\in N\Rightarrow\left(x+1\right)^2+\left(y+3\right)^2\ge0\)
\(\Rightarrow B_{min}\Leftrightarrow\left(x+1\right)^2+\left(y+3\right)^2=0\Rightarrow B_{min}=1\)
\(A=\frac{3}{\left(x+2\right)^2+4}\)
Để A max
=>(x+2)^2+4 min
Mà\(\left(x+2\right)^2\ge0\Rightarrow\left(x+2\right)^2+4\ge4\)
Vậy Min = 4 <=>x=-2
Vậy Max A = 3/4 <=> x=-2
\(b,B=\left(x+1\right)^2+\left(y+3\right)^2+1\)
Có \(\left(x+1\right)^2\ge0;\left(y+3\right)^2\ge0\)
\(\Rightarrow B\ge0+0+1=1\)
Vậy MinB = 1<=>x=-1;y=-3
Ta có: \(7-x^2-y^2-2\left(x+y\right)\)
\(=7-x^2-y^2-2x-2y\)
\(=-1-1+9-x^2-y^2-2x-2y\)
\(=\left(-x^2-2x-1\right)+\left(-y^2-2y-1\right)+9\)
\(=-\left(x^2+2x+1\right)-\left(y^2+2y+1\right)+9\)
\(=-\left(x+1\right)^2-\left(y+1\right)^2+9\)
\(\text{Vì}-\left(x+1\right)^2\le0\)
\(\text{và}-\left(y+1\right)^2\le0\)
\(\Rightarrow-\left(x+1\right)^2-\left(y+1\right)^2\le0\)
\(\Rightarrow-\left(x+1\right)^2-\left(y+1\right)^2+9\le9\)
\(\text{Vậy GTLN = 9, dấu bằng xảy ra khi x = -1 và y = -1}\)