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Đặt A =\(a^2+b^2+\left(\frac{ab+1}{a+b}\right)^2\)
Vì a + b \(\ne\)0 nên A luôn được xác định.
Giả sử \(a^2+b^2+\left(\frac{ab+1}{a+b}\right)^2\ge2\)
\(\Leftrightarrow\frac{\left(a^2+b^2\right)\left(a+b\right)^2}{\left(a+b\right)^2}+\frac{\left(ab+1\right)^2}{\left(a+b\right)^2}-\frac{2\left(a+b\right)^2}{\left(a+b\right)^2}\ge0\)
\(\Leftrightarrow\left(a^2+b^2\right)\left(a+b\right)^2+\left(ab+1\right)^2-2\left(a+b\right)^2\ge0\)(vì a + b \(\ne\)0)
\(\Leftrightarrow[\left(a^2+2ab+b^2\right)-2ab]\left(a+b\right)^2+\left(ab+1\right)^2-2\left(a+b\right)^2\ge0\)
\(\Leftrightarrow[\left(a+b\right)^2-2ab]\left(a+b\right)^2+\left(ab+1\right)^2-2\left(a+b\right)^2\ge0\)
\(\Leftrightarrow\left(a+b\right)^4-2ab\left(a+b\right)^2+\left(ab+1\right)^2-2\left(a+b\right)^2\ge0\)
\(\Leftrightarrow\left(a+b\right)^4-\left[2ab\left(a+b\right)^2+2\left(a+b\right)^2\right]+\left(ab+1\right)^2\ge0\)
\(\Leftrightarrow\left[\left(a+b\right)^2\right]^2-2\left(a+b\right)^2\left(ab+1\right)+\left(ab+1\right)^2\ge0\)
\(\left[\left(a+b\right)^2-\left(ab+1\right)^2\right]^2\ge0\)(luôn đúng)
Dấu bằng xảy ra
\(\Leftrightarrow\hept{\begin{cases}a+b\ne0\\\Leftrightarrow a=b\end{cases}}\Leftrightarrow a=b\left(a,b\ne0\right)\)
Vậy \(a^2+b^2+\left(\frac{ab+1}{a+b}\right)^2\ge\)2 với a, b là các số thỏa mãn a+b \(\ne\)0
Dấu bằng xảy ra
\(\Leftrightarrow\hept{\begin{cases}a=b\\a+b\ne0\end{cases}\Leftrightarrow a=b}\)(a,b \(\ne\)0)
Vậy \(a^2+b^2+\left(\frac{ab+1}{a+b}\right)^2\ge2\) với a, b là các số thỏa mãn \(a+b\ne0\)
a)Có \(a^2+1\ge2a\) với mọi a; \(b^2+1\ge2b\) với mọi b
Cộng vế với vế \(\Rightarrow a^2+b^2+2\ge2\left(a+b\right)\)
Dấu = xảy ra <=> a=b=1
b) Áp dụng BĐT bunhiacopxki có:
\(\left(x+y\right)^2\le\left(1+1\right)\left(x^2+y^2\right)\Leftrightarrow\left(x+y\right)^2\le2\)
\(\Leftrightarrow-\sqrt{2}\le x+y\le\sqrt{2}\)
\(\Rightarrow\left(x+y\right)_{max}=\sqrt{2}\Leftrightarrow\left\{{}\begin{matrix}x+y=\sqrt{2}\\x=y\end{matrix}\right.\)\(\Leftrightarrow x=y=\dfrac{\sqrt{2}}{2}\)
\(\left(x+y\right)_{min}=-\sqrt{2}\Leftrightarrow\left\{{}\begin{matrix}x+y=-\sqrt{2}\\x=y\end{matrix}\right.\)\(\Leftrightarrow x=y=-\dfrac{\sqrt{2}}{2}\)
c) \(S=\dfrac{1}{ab}+\dfrac{1}{a^2+b^2}=\dfrac{1}{a^2+b^2}+\dfrac{1}{2ab}+\dfrac{1}{2ab}\)
Với x,y>0, ta có: \(\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{4}{x+y}\) (1)
Thật vậy (1) \(\Leftrightarrow\dfrac{y+x}{xy}\ge\dfrac{4}{x+y}\Leftrightarrow\left(x+y\right)^2\ge4xy\)\(\Leftrightarrow\left(x-y\right)^2\ge0\) (lđ)
Áp dụng (1) vào S ta được:
\(S\ge\dfrac{4}{a^2+b^2+2ab}+\dfrac{1}{2ab}\)
Lại có: \(ab\le\dfrac{\left(a+b\right)^2}{4}\) \(\Leftrightarrow2ab\le\dfrac{\left(a+b\right)^2}{2}\Leftrightarrow2ab\le\dfrac{1}{2}\)\(\Rightarrow\dfrac{1}{2ab}\ge2\)
\(\Rightarrow S\ge\dfrac{4}{\left(a+b\right)^2}+2=6\)
\(\Rightarrow S_{min}=6\Leftrightarrow a=b=\dfrac{1}{2}\)
Ta có: \(a^2+b^2+\left(\frac{ab+1}{a+b}\right)^2\ge2\)
\(\Leftrightarrow\left(a^2+b^2\right)\left(a+b\right)^2+\left(ab+1\right)^2\ge2\left(a+b\right)^2\)
\(\Leftrightarrow\left(a+b\right)^2\left[\left(a+b\right)^2-2ab\right]-2\left(a+b\right)^2+\left(ab+1\right)^2\ge0\)
\(\Leftrightarrow\left(a+b\right)^4-2ab\left(a+b\right)^2-2\left(a+b\right)^2+\left(ab+1\right)^2\ge0\)
\(\Leftrightarrow\left[\left(a+b\right)^2-ab-1\right]^2\ge0\)(đúng)
\(\Leftrightarrow dpcm\)
⇔(a2+b2)(a+b)2+(ab+1)2≥2(a+b)2
⇔(a+b)2[(a+b)2−2ab]−2(a+b)2+(ab+1)2≥0
⇔(a+b)4−2ab(a+b)2−2(a+b)2+(ab+1)2≥0
⇔[(a+b)2−ab−1]2≥0(đúng)
k mình đi
Lời giải:
$a^2+b^2+c^2-ab-bc-ac=0$
$\Leftrightarrow 2a^2+2b^2+2c^2-2ab-2bc-2ac=0$
$\Leftrightarrow (a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ac+a^2)=0$
$\Leftrightarrow (a-b)^2+(b-c)^2+(c-a)^2=0$
Vì $(a-b)^2; (b-c)^2; (c-a)^2\geq 0$ với mọi $a,b,c$ nên để tổng của chúng bằng $0$ thì:
$a-b=b-c=c-a=0$
$\Rightarrow a=b=c$
$\Rightarrow \frac{a}{b}=\frac{b}{c}=\frac{c}{a}=1$
Khi đó:
$(\frac{a}{b}+1)(\frac{b}{c}+1)(\frac{c}{a}+1)=(1+1)(1+1)(1+1)=8$
Ta có đpcm.
BĐT tương đương
\(a^2+b^2+\frac{a^2b^2+2ab+1}{\left(a+b\right)^2}\ge2\)
<=>\(\left(a+b\right)^2-2+\frac{1}{\left(a+b\right)^2}+\frac{a^2b^2}{\left(a+b\right)^2}+\frac{2ab}{\left(a+b\right)^2}-2ab\ge0\)
<=>\(\left(a+b\right)^2-2.\left(a+b\right).\frac{1}{a+b}+\frac{a^2b^2}{\left(a+b\right)^2}-2.\left(ab-\frac{ab}{\left(a+b\right)^2}\right)\ge0\)
<=>\(\left(a+b-\frac{1}{a+b}\right)^2+\frac{a^2b^2}{\left(a+b\right)^2}-2.\left(\frac{ab\left(a+b\right)^2-ab}{\left(a+b\right)^2}\right)\ge0\)
<=>\(\left(\frac{\left(a+b\right)^2-1}{a+b}\right)^2+\frac{a^2b^2}{\left(a+b\right)^2}-2.\left(\frac{ab\left[\left(a+b\right)^2-1\right]}{\left(a+b\right)\left(a+b\right)}\right)\ge0\)
<=>\(\left(\frac{\left(a+b\right)^2-1}{a+b}\right)^2+\frac{a^2b^2}{\left(a+b\right)^2}-2.\frac{\left(a+b\right)^2-1}{a+b}.\frac{ab}{a+b}\ge0\)
<=>\(\left(\frac{\left(a+b\right)^2-1}{a+b}-\frac{ab}{a+b}\right)^2\ge0\left(\text{luôn đúng}\right)\)
=> dpcm
\(\frac{1}{a^2}=\frac{1}{\left(bc\right)^2}\)
\(\Rightarrow\frac{1}{a^2}+1=\frac{1}{\left(bc\right)^2}+1\ge2\frac{1}{bc}=2a\)
Bài 2 :
\(\left(a+b+c\right)^2=3\left(ab+bc+ca\right)\)
<=> a^2 + b^2 + c^2 + 2ab + 2bc + 2ca = 3ab + 3bc + 3ca
<=> a^2 + b^2 + c^2 = ab + bc + ca
<=> 2a^2 + 2b^2 + 2c^2 = 2ab + 2bc + 2ca
<=> ( a - b )^2 + ( b - c )^2 + ( c - a )^2 = 0
<=> a = b = c
1.
\(\Leftrightarrow2a^2+2b^2+18=2ab+6a+6b\)
\(\Leftrightarrow\left(a^2-2ab+b^2\right)+\left(a^2-6a+9\right)+\left(b^2-6b+9\right)=0\)
\(\Leftrightarrow\left(a-b\right)^2+\left(a-3\right)^2+\left(b-3\right)^2=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}a-b=0\\a-3=0\\b-3=0\end{matrix}\right.\) \(\Leftrightarrow a=b=3\)
2.
\(\left(a+b+c\right)^2=3\left(ab+bc+ca\right)\)
\(\Leftrightarrow a^2+b^2+c^2+2ab+2bc+2ca=3ab+3bc+3ca\)
\(\Leftrightarrow2a^2+2b^2+2c^2-2ab-2bc-2ca=0\)
\(\Leftrightarrow\left(a^2-2ab+b^2\right)+\left(b^2-2bc+c^2\right)+\left(c^2-2ca+a^2\right)=0\)
\(\Leftrightarrow\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}a-b=0\\b-c=0\\c-a=0\end{matrix}\right.\) \(\Leftrightarrow a=b=c\)
