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\(\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}=\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2-2.\left(\dfrac{1}{ab}+\dfrac{1}{bc}+\dfrac{1}{ca}\right)=\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2-2.\dfrac{a+b+c}{abc}=\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2-2.\dfrac{0}{abc}=\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2\)
Do \(a-b+b-c+c-a=0\)
\(\Rightarrow2\dfrac{\left(a-b\right)+\left(b-c\right)+\left(c-a\right)}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}=0\)
\(\Rightarrow\dfrac{2}{\left(a-b\right)\left(b-c\right)}+\dfrac{2}{\left(a-b\right)\left(c-a\right)}+\dfrac{2}{\left(b-c\right)\left(c-a\right)}=0\)
\(\Rightarrow N=\dfrac{1}{\left(a-b\right)^2}+\dfrac{1}{\left(b-c\right)^2}+\dfrac{1}{\left(c-a\right)^2}+0\)
\(\Rightarrow N=\dfrac{1}{\left(a-b\right)^2}+\dfrac{1}{\left(b-c\right)^2}+\dfrac{1}{\left(c-a\right)^2}+\dfrac{2}{\left(a-b\right)\left(b-c\right)}+\dfrac{2}{\left(a-b\right)\left(c-a\right)}+\dfrac{2}{\left(b-c\right)\left(c-a\right)}\)
\(\Rightarrow N=\left(\dfrac{1}{a-b}+\dfrac{1}{a-c}+\dfrac{1}{b-c}\right)^2\) (đpcm)
\(\dfrac{1}{ab}+\dfrac{1}{bc}+\dfrac{1}{ca}=\dfrac{a}{abc}+\dfrac{b}{abc}+\dfrac{c}{abc}=\dfrac{a+b+c}{abc}=0\left(a+b+c=0\right)\\ \Rightarrow\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}=\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}+2\left(\dfrac{1}{ab}+\dfrac{1}{bc}+\dfrac{1}{ca}\right)=\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2\)
\(\)1 a b + 1 b c + 1 c a = a a b c + b a b c + c a b c = a + b + c a b c = 0 ( a + b + c = 0 ) ⇒ 1 a 2 + 1 b 2 + 1 c 2 = 1 a 2 + 1 b 2 + 1 c 2 + 2 ( 1 a b + 1 b c + 1 c a ) = ( 1 a + 1 b + 1 c ) 2
Từ \(a+b+c=0\Rightarrow a+b=-c\)
\(\Rightarrow\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}=\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{\left(a+b\right)^2}=\dfrac{b^2\left(a+b\right)^2+a^2\left(a+b\right)^2+\left(ab\right)^2}{a^2b^2\left(a+b\right)^2}\)
\(=\dfrac{a^2b^2+2ab^3+b^4+a^4+2a^3b+a^2b^2+a^2b^2}{a^2b^2\left(a+b\right)^2}\)
\(=\dfrac{a^4+2ab^3+2a^3b+3a^2b^2+b^4}{a^2b^2\left(a+b\right)^2}\)
\(=\dfrac{\left(a^2+ab+b^2\right)^2}{a^2b^2\left(a+b\right)^2}=\left[\dfrac{a^2+ab+b^2}{ab\left(a+b\right)}\right]^2\) là bình phương của 1 số hữu tỉ (đpcm)
đánh mỏi tay đó ))))
\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{2016}\)
\(\Rightarrow\dfrac{bc+ac+bc}{abc}=\dfrac{1}{2016}\)
\(\Rightarrow\dfrac{bc+ac+ab}{abc}=\dfrac{1}{a+b+c}\)
\(\Rightarrow\left(ab+bc+ca\right)\left(a+b+c\right)=abc\)
\(\Rightarrow ab\left(a+b\right)+bc\left(b+c\right)+ca\left(c+a\right)+3abc=abc\)
\(\Rightarrow ab\left(a+b\right)+bc\left(b+c\right)+ca\left(c+a\right)+2abc=0\)
\(\Rightarrow\left(a+b\right)\left(b+c\right)\left(c+a\right)=0\)
\(\Rightarrow a=-b\) hay \(b=-c\) hay \(c=-a\)
-Vậy trong ba số a,b,c tồn tại 2 số đối nhau.
25 ban oi
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