a)a²+b²+c²+3=2(a+b+c)

=>a²+b²+c²+1+1+1-2a-2b-2c=0

=>(a²-2a+1)+(b²-2b+1)+(c²-2c+1)=0

=>(a-1)²+(b-1)²+(c-1)²=0

=>a-1=b-1=c-1=0 <=>a=b=c=1 

-->Đpcm

b)(a+b+c)²=3(ab+ac+bc)

=>a²+b²+c²+2ab+2ac+2bc -3ab-3ac-3bc=0 

=>a²+b²+c²-ab-ac-bc=0

=>2a²+2b²+2c²-2ab-2ac-2bc=0 

=>(a²- 2ab+b²)+(b²-2bc+c²) + (c²-2ca+a²) = 0

=>(a-b)²+(b-c)²+(c-a)²=0 

Hay (a-b)²=0 hoặc (b-c)²=0 hoặc (a-c)²=0

=>a-b hoặc b=c hoặc a=c

=>a=b=c 

-->Đpcm

c)a²+b²+c²=ab+bc+ca

=>2(a²+b²+c²)=2(ab+bc+ca)

=>2a²+2b²+c²=2ab+2bc+2ca

=>2a²+2b²+c²-2ab-2bc-2ca=0

=>a²+a²+b²+b²+c²+c²-2ab-2bc-2ca=0

=>(a²-2ab+b²)+(b²-2bc+c²)+(a²-2ca+c²)=0

=>(a-b)²+(b-c)²+(a-c)²=0

Hay (a-b)²=0 hoặc (b-c)²=0 hoặc (a-c)²=0

=>a-b hoặc b=c hoặc a=c

=>a=b=c 

-->Đpcm

a) Ta có : \(a^2+b^2+c^2+3=2\left(a+b+c\right)\)

\(\Leftrightarrow\left(a^2-2a+1\right)+\left(b^2-2b+1\right)+\left(c^2-2c+1\right)=0\)

\(\Leftrightarrow\left(a-1\right)^2+\left(b-1\right)^2+\left(c-1\right)^2=0\)

Vì \(\left(a-1\right)^2\ge0,\left(b-1\right)^2\ge0,\left(c-1\right)^2\ge0\) nên pt trên tương đương với \(\begin{cases}\left(a-1\right)^2=0\\\left(b-1\right)^2=0\\\left(c-1\right)^2=0\end{cases}\) \(\Leftrightarrow a=b=c=1\)

b) \(\left(a+b+c\right)^2=3\left(ab+bc+ac\right)\)

\(\Leftrightarrow a^2+b^2+c^2+2\left(ab+bc+ac\right)=3\left(ab+bc+ac\right)\)

\(\Leftrightarrow a^2+b^2+c^2=ab+bc+ac\) (1)

\(\Leftrightarrow2\left(a^2+b^2+c^2\right)=2ab+2bc+2ac\)

\(\Leftrightarrow\left(a^2-2ab+b^2\right)+\left(b^2-2bc+c^2\right)+\left(c^2-2ac+a^2\right)=0\)

\(\Leftrightarrow\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)

Mà \(\left(a-b\right)^2\ge0,\left(b-c\right)^2\ge0,\left(c-a\right)^2\ge0\)

\(\Rightarrow\begin{cases}\left(a-b\right)^2=0\\\left(b-c\right)^2=0\\\left(c-a\right)^2=0\end{cases}\) \(\Rightarrow a=b=c\)

c) Giải tương tự câu b) , bắt đầu từ (1)

a. \(2\left(a^2+b^2\right)=\left(a-b\right)^2\)

\(\Leftrightarrow2a^2+2b^2=a^2+b^2-2ab\)

\(\Leftrightarrow a^2+b^2=-2ab\)

\(\Leftrightarrow a^2+2ab+b^2=0\)

\(\Leftrightarrow\left(a+b\right)^2=0\)

\(\Leftrightarrow a+b=0\Leftrightarrow a=-b\) (đpcm)

b. \(a^2+b^2+c^2+3=2\left(a+b+c\right)\)

\(\Leftrightarrow a^2+b^2+c^2+3-2a-2b-2c=0\)

\(\Leftrightarrow\left(a^2-2a+1\right)+\left(b^2-2b+1\right)+\left(c^2-2c+1\right)=0\)

\(\Leftrightarrow\left(a-1\right)^2+\left(b-1\right)^2+\left(c-1\right)^2=0\)

Vì \(\left(a-1\right)^2;\left(b-1\right)^2;\left(c-1\right)^2\ge0\)

\(\Rightarrow\left(a-1\right)^2=\left(b-1\right)^2=\left(c-1\right)^2=0\)

\(\Leftrightarrow a-1=b-1=c-1=0\Leftrightarrow a=b=c=1\)

c. \(\left(a+b+c\right)^2=3\left(ab+bc+ca\right)\)

\(\Leftrightarrow a^2+b^2+c^2+2\left(ab+bc+ca\right)=3\left(ab+bc+ca\right)\)

\(\Leftrightarrow a^2+b^2+c^2=ab+bc+ca\)

\(\Leftrightarrow2\left(a^2+b^2+c^2\right)=2\left(ab+bc+ca\right)\)

\(\Leftrightarrow\left(a^2-2ab+b^2\right)+\left(b^2-2bc+c^2\right)+\left(c^2-2ca+a^2\right)=0\)

\(\Leftrightarrow\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)

Tương tự câu b ta có a = b = c

Đặt \(\frac{a}{b}=\frac{c}{d}=t\Rightarrow a=bt;c=dt\)

Thay vào từng vế ta có 

     \(\frac{a.b}{c.d}=\frac{bt.b}{dt.d}=\frac{b^2.t}{d^2.t}=\frac{b^2}{d^2}\) (1)

     \(\frac{\left(bt+b\right)^2}{\left(dt+d\right)^2}=\frac{b^2\left(t+1\right)^2}{d^2\left(t+1\right)^2}=\frac{b^2}{d^2}\) (2)

Từ (1) và (2) => ĐPCM

a/b=c/d 
=> a/c = b/d
Áp dụng tính chất dãy tỉ số bằng nhau có : 
a/c = b/d = a+b/c+d
=> (a/c)mũ 2 = (b/d)mũ 2 = a/c.b/d= ( a+b/c+d ) mũ 2 
=>   a/c.b/d= ( a+b/c+d ) mũ 2 
=> a.b/c.d = (a+b)mũ 2 / (c + d ) mũ 2 
=> dpcm

Ta có a/b = c/d 

 => a/c= b/d 

adtccdtsbn ta có : 

\(https://scontent.fhph1-1.fna.fbcdn.net/v/t34.0-12/19987311_122536408488931_1351154453_n.jpg?oh=553755e5363013e1853ab6f5ed63a600&oe=59BF5CA7\)https://scontent.fhph1-1.fna.fbcdn.net/v/t34.0-12/19987311_122536408488931_1351154453_n.jpg?oh=553755e5363013e1853ab6f5ed63a600&oe=59BF5CA7
Ấn vào linh đấy ế

1a)Xét a² + 5 - 4a =a² - 4a + 4+1=(a - 2)²+1\(\ge\)1 hay (a -2)² + 1 > 0 

\(\Rightarrow\)Đpcm

  b)Xét 3(a² + b² + c²) -(a + b +c)² =3a² + 3b² + 3c² - a² - b² - c² - 2ab - 2ac - 2bc

                                                  =2a² + 2b² + 2c²  - 2ab - 2ac - 2bc

                                                  =(a - b)² + (a - c)² + (b - c)²\(\ge\)0 (với mọi a,b,c)

\(\Rightarrow\)Đpcm

2)Xét A=\(\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\left(a+c+b\right)=3+\frac{a}{b}+\frac{a}{c}+\frac{b}{a}+\frac{b}{c}+\frac{c}{a}+\frac{c}{b}\)

         áp dụng cô-sy

\(\Rightarrow\)A\(\ge\)9

\(\Rightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge\frac{9}{a+b+c}=3\)

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