Cho a,b,c>0, ab+bc+ac=3abc
Và\(\frac{1}{1+a^2}\)+ \(\frac{1}{1+b^2}\)+ \(\frac{1}{1+c^2}\)= \(\frac{3}{2}\)
Tính S= a+b+c
Cho a,b,c thỏa mãn:\(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\) tính A=\(\frac{bc}{a^2}+\frac{ac}{b^2}+\frac{ab}{c^2}\)
\(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\) <=> \(\frac{1}{a}+\frac{1}{b}=-\frac{1}{c}\)
<=> \(\left(\frac{1}{a}+\frac{1}{b}\right)^3=\left(-\frac{1}{c}\right)^3\)
<=> \(\frac{1}{a^3}+\frac{1}{b^3}+\frac{3}{a^2b}+\frac{3}{ab^2}=-\frac{1}{c^3}\)
<=> \(\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}=-\frac{3}{ab}\left(\frac{1}{a}+\frac{1}{b}\right)\)
<=> \(\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}=\frac{3}{abc}\)
Khi đó, A = \(\frac{bc}{a^2}+\frac{ac}{b^2}+\frac{ab}{c^2}=abc\left(\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}\right)=abc\cdot\frac{3}{abc}=3\)
Xét: \(A=\frac{bc}{a^2}+\frac{ac}{b^2}+\frac{ab}{c^2}=abc\left(\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}\right)\)
Ta có đẳng thức sau: \(x^3+y^3+z^3=\left(x+y+z\right)\left(x^2+y^2+z^2-xy-yz-xz\right)+3xyz\)
(Đẳng thức này chứng minh rất dễ nha, chỉ cần bung hết ra là được)
Vậy ta thế \(x=\frac{1}{a},y=\frac{1}{b},z=\frac{1}{c}\)vào đẳng thức:
\(\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}=\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\left(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}-\frac{1}{ab}-\frac{1}{bc}-\frac{1}{ca}\right)+\frac{3}{abc}\)
\(=\frac{3}{abc}\)Vì \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)---> Thế cái này vào A:
\(\Rightarrow A=abc.\frac{3}{abc}=3\)
Xoooooooong !!!!! :)))
Cho abc=8 và \(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}=\frac{3}{4}\)(a,b,c>0). Tính giá trị của: \(\frac{bc}{a}+\frac{ac}{b}+\frac{ab}{c}\)
\(\left(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}\right)abc=\frac{3}{4}8\Rightarrow\frac{abc}{a^2}+\frac{abc}{b^2}+\frac{abc}{c^2}=\frac{3.8}{4}\Leftrightarrow\)\(\frac{bc}{a}+\frac{ac}{b}+\frac{ab}{c}=6\)
\(\frac{a^2b+bc^2-1}{ac\left(a+c\right)}+\frac{b^2c+ca^2-1}{ab\left(a+b\right)}+\frac{c^2a+ab^2-1}{bc\left(b+c\right)}\)
\(=\frac{a^2b^2+b^2c^2-b}{a+c}+\frac{b^2c^2+c^2a^2-c}{a+b}+\frac{c^2a^2+a^2b^2-a}{b+c}\)
\(=\frac{\frac{1}{a^2}-\frac{1}{ac}+\frac{1}{c^2}}{a+c}+\frac{\frac{1}{b^2}-\frac{1}{ab}+\frac{1}{a^2}}{a+b}+\frac{\frac{1}{c^2}-\frac{1}{bc}+\frac{1}{b^2}}{b+c}\ge\frac{1}{ac\left(a+c\right)}+\frac{1}{bc\left(b+c\right)}+\frac{1}{ab\left(b+a\right)}\)
\(=\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\ge\frac{3}{2}\)
Cho a,b,c là 3 số đôi một khác nhau và khác 0 thỏa mãn \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)
Tính giá trị của biểu thức M=\(\frac{1}{a^2+bc}+\frac{1}{b^2+ac}+\frac{1}{c^2+ab}\)
Tham khảo: Câu hỏi của Nguyễn Thị Nhàn - Toán lớp 8 - Học toán với OnlineMath
Học tốt=)
tth : mẫu nó khác bạn nhé
- mẫu nó là 2bc 2ac 2ab
mẫu mk ko có nhân 2
cho a>0, b>0, c>0, a+b+c=1
tìm max của S=\(\frac{1}{a^2+b^2+c^2}+\frac{1}{ab}+\frac{1}{ac}+\frac{1}{bc}\)
cho a>0, b>0, c>0, a+b+c=1
tìm min của S=\(\frac{1}{a^2+b^2+c^2}+\frac{1}{ab}+\frac{1}{ac}+\frac{1}{bc}\)
Cho 3 số a,b,c khác 0 thỏa mãn \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)
\(\text{Tính: }P=\frac{ab}{c^2}+\frac{bc}{a^2}+\frac{ac}{b^2}\)
Cho a,b,c đôi một khác nhau và \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)
Tính \(A=\frac{bc+1}{a^2+2bc}+\frac{ac+1}{b^2+2ac}+\frac{ab+1}{c^2+2ab}\)
\(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)
\(\Leftrightarrow\)\(\frac{bc+ac+ab}{abc}=0\)
\(\Leftrightarrow\)\(bc+ac+ab=0\)
\(\Leftrightarrow\)\(\hept{\begin{cases}bc=-ab-ac\\ac=-ab-bc\\ab=-bc-ac\end{cases}}\)\(\Leftrightarrow\)\(\hept{\begin{cases}a^2+2bc=a^2+bc-ab-ac=\left(a-b\right)\left(a-c\right)\\b^2+2ac=b^2+ac-ab-bc=\left(b-a\right)\left(b-c\right)\\c^2+2ab=c^2+ab-bc-ac=\left(c-a\right)\left(c-b\right)\end{cases}}\)
\(A=\frac{bc+1}{\left(a-b\right)\left(a-c\right)}+\frac{ac+1}{\left(b-a\right)\left(b-c\right)}+\frac{ab+1}{\left(c-a\right)\left(c-b\right)}\)
= \(\frac{bc\left(b-c\right)+b-c+ac\left(c-a\right)+c-a+ab\left(a-b\right)+a-b}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)
= \(\frac{bc\left(b-c\right)+ca\left(c-a\right)-ab\left(b-c\right)-ab\left(c-a\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)
= \(\frac{\left(b-c\right)\left(bc-ab\right)+\left(c-a\right)\left(ca-ab\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)
= \(\frac{b\left(b-c\right)\left(c-a\right)+a\left(c-a\right)\left(c-b\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)
= \(\frac{\left(a-c\right)\left(b-c\right)\left(a-b\right)}{\left(a-b\right)\left(a-c\right)\left(b-c\right)}=1\)
Cho \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)
Tính \(P=\frac{ab}{c^2}+\frac{bc}{a^2}+\frac{ac}{b^2}\)
P= abc(\(\frac{1}{^{a^3}}\)+\(\frac{1}{b^3}\)+\(\frac{1}{c^3}\)) = abc[(\(\frac{1}{a}\)+\(\frac{1}{b}\))3+\(\frac{1}{c^3}\)-\(\frac{3}{a^2b}\)-\(\frac{3}{ab^2}\)]=abc[(\(\frac{1}{a}\)+\(\frac{1}{b}\)+\(\frac{1}{c}\))(....)- \(\frac{3}{a^2b}\)-\(\frac{3}{ab^2}\)]
=abc.(- \(\frac{3}{a^2b}\)-\(\frac{3}{ab^2}\)) =-3(\(\frac{c}{a}\)+\(\frac{c}{b}\)) = -3c(\(\frac{1}{a}\)+\(\frac{1}{b}\)) = -3c.\(\frac{-1}{c}\)=3
P = 3
Đầu tiên,bạn cần chứng minh x + y + z = 0 thì x3 + y3 + z3 = 3xyz ( Bạn ko biết c/m thì hỏi nhé)
Thay\(x=\frac{1}{a};y=\frac{1}{b};z=\frac{1}{c}\Rightarrow\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}=3.\frac{1}{a}.\frac{1}{b}.\frac{1}{c}=\frac{3}{abc}\)
\(\Rightarrow M=\frac{ab}{c^2}+\frac{bc}{a^2}+\frac{ac}{b^2}=\frac{abc}{c^3}+\frac{abc}{a^3}+\frac{abc}{b^3}=abc\left(\frac{1}{c^3}+\frac{1}{a^3}+\frac{1}{b^3}\right)=abc.\frac{3}{abc}=3\)