\(a+b+c=3abc\Rightarrow\dfrac{1}{bc}+\dfrac{1}{ac}+\dfrac{1}{ab}=3\)
\(\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2=9\)
\(\Leftrightarrow\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}+\dfrac{2}{ab}+\dfrac{2}{bc}+\dfrac{2}{ac}=9\)
\(\Leftrightarrow\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}+2\cdot3=9\)
Vậy \(\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}=3\).
Cho\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a+b+c}\)
Chứng minh:\(\dfrac{1}{a^3+b^3+c^3}=\dfrac{1}{a^3}+\dfrac{1}{b^3}+\dfrac{1}{c^3}\)
BT1: Cho a+b>1. Chứng minh: a4+b4>=\(\dfrac{1}{8}\)
BT2: Cho a,b,c>0. Chứng minh rằng: \(\dfrac{a^3}{b^2}+\dfrac{b^3}{c^2}+\dfrac{c^3}{a^2}>=a+b+c\)
Cho \(\dfrac{x}{a}+\dfrac{y}{b}+\dfrac{z}{c}=1\) và \(\dfrac{a}{x}+\dfrac{b}{y}+\dfrac{c}{z}=0\)
Chứng minh rằng :\(\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}=1\)
bài 1:
Cho \(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a+b+c}\) .Chứng minh rằng:
\(\dfrac{1}{a^{1995}}+\dfrac{1}{b^{1995}}+\dfrac{1}{c^{1995}}=\dfrac{1}{a^{1995}+b^{1995}+c^{1995}}\)
Bài 1: Cho \(\text{a+b+c=ab+bc+ac=abc}\) \(\ne\) \(0\) và \(\dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}=2\)
Tính \(A=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\)
Bài 2: Cho \(a,b,c\ne0\). CMR nếu \(x,y\) thỏa mãn :
\(\dfrac{a}{c}x+\dfrac{b}{c}y=\dfrac{b}{a}x+\dfrac{c}{a}y=\dfrac{c}{b}x+\dfrac{a}{b}y=1\)
thì \(\dfrac{a^2}{bc}+\dfrac{b^2}{ac}+\dfrac{c^2}{ab}=3\)
Bài 3: Cho \(ax+by+cz=0\) và \(a+b+c=\dfrac{1}{2019}\)
Tính \(A=\dfrac{a^2x^2+b^2y^2+c^2z^2}{bc\left(y-z\right)^2+ac\left(x-z\right)^2+ab\left(x-y\right)^2}\)
cho \(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a+b+c}\)
CMR \(\dfrac{1}{a^3}+\dfrac{1}{b^3}+\dfrac{1}{c^3}=\dfrac{1}{a^3+b^3+c^3}\)
Cho a,b,c dương thỏa mãn abc=1
CMR : \(P=\dfrac{a^2}{1+b}+\dfrac{b^2}{1+c}+\dfrac{c^2}{a+1}\ge\dfrac{3}{2}\)
cho A=\(\dfrac{1}{1.2^2}+\dfrac{1}{2.3^2}+...+\dfrac{1}{49.50^2}\)và B=\(\dfrac{1}{2^2}+\dfrac{1}{3^2}+...+\dfrac{1}{50^2}\),Chứng minh A<\(\dfrac{1}{2}\)<B
Bài 2 :
a, Cho các số a,b,c,d là các số nguyên dương đôi 1 khác nhau và thỏa mãn :
\(\dfrac{2a+b}{a+b}+\dfrac{2b+c}{b+c}+\dfrac{2c+d}{c+d}+\dfrac{2d+a}{d+a}=6\) . Chứng minh \(A=abcd\) là số chính phương
b, Tìm nguyên a để \(a^3-2a^2+7a-7\) chia hết cho \(a^2+3\)
