Cho : \(\dfrac{a}{c}=\dfrac{c}{b}\) CMR :
a.\(\dfrac{a^2+c^2}{b^2+c^2}=\dfrac{a}{b}\)
b.\(\dfrac{b^2-a^2}{a^2+c^2}=\dfrac{b-a}{a}\)
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Những câu hỏi liên quan
HP
30 tháng 1 2021
1.
Áp dụng BĐT BSC:
\(\dfrac{a^2}{b+c}+\dfrac{b^2}{c+a}+\dfrac{c^2}{a+b}\ge\dfrac{\left(a+b+c\right)^2}{2\left(a+b+c\right)}=\dfrac{a+b+c}{2}\)
Đẳng thức xảy ra khi \(a=b=c>0\)
2.
Áp dụng BĐT \(x^2+y^2\ge\dfrac{\left(x+y\right)^2}{2}\) và BĐT BSC:
\(\dfrac{a+b}{a^2+b^2}+\dfrac{b+c}{b^2+c^2}+\dfrac{c+a}{c^2+a^2}\)
\(\le\dfrac{a+b}{\dfrac{\left(a+b\right)^2}{2}}+\dfrac{b+c}{\dfrac{\left(b+c\right)^2}{2}}+\dfrac{c+a}{\dfrac{\left(c+a\right)^2}{2}}\)
\(=\dfrac{2}{a+b}+\dfrac{2}{b+c}+\dfrac{2}{c+a}\)
\(\le2.\dfrac{1}{4}\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{b}+\dfrac{1}{c}+\dfrac{1}{c}+\dfrac{1}{a}\right)\)
\(=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\)
Đẳng thức xảy ra khi \(a=b=c>0\)
HP
30 tháng 1 2021
Cách khác:
1.
Áp dụng BĐT Cauchy:
\(\dfrac{a^2}{b+c}+\dfrac{b+c}{4}+\dfrac{b^2}{c+a}+\dfrac{c+a}{4}+\dfrac{c^2}{a+b}+\dfrac{a+b}{4}\ge a+b+c\)
\(\Leftrightarrow\dfrac{a^2}{b+c}+\dfrac{b^2}{c+a}+\dfrac{c^2}{a+b}\ge a+b+c-\dfrac{a+b+c}{2}=\dfrac{a+b+c}{2}\)
Đẳng thức xảy ra khi \(a=b=c>0\)
3 tháng 1 2019
3/ Áp dụng bất đẳng thức AM-GM, ta có :
\(\dfrac{a^2}{b^2}+\dfrac{b^2}{c^2}\ge2\sqrt{\dfrac{\left(ab\right)^2}{\left(bc\right)^2}}=\dfrac{2a}{c}\)
\(\dfrac{b^2}{c^2}+\dfrac{c^2}{a^2}\ge2\sqrt{\dfrac{\left(bc\right)^2}{\left(ac\right)^2}}=\dfrac{2b}{a}\)
\(\dfrac{c^2}{a^2}+\dfrac{a^2}{b^2}\ge2\sqrt{\dfrac{\left(ac\right)^2}{\left(ab\right)^2}}=\dfrac{2c}{b}\)
Cộng 3 vế của BĐT trên ta có :
\(2\left(\dfrac{a^2}{b^2}+\dfrac{b^2}{c^2}+\dfrac{c^2}{a^2}\right)\ge2\left(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\right)\)
\(\Leftrightarrow\dfrac{a^2}{b^2}+\dfrac{b^2}{c^2}+\dfrac{c^2}{a^2}\ge\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}\left(\text{đpcm}\right)\)
AH
Akai Haruma
Giáo viên
4 tháng 1 2019
Bài 1:
Áp dụng BĐT AM-GM ta có:
\(\frac{1}{a^2+bc}+\frac{1}{b^2+ac}+\frac{1}{c^2+ab}\leq \frac{1}{2\sqrt{a^2.bc}}+\frac{1}{2\sqrt{b^2.ac}}+\frac{1}{2\sqrt{c^2.ab}}=\frac{\sqrt{ab}+\sqrt{bc}+\sqrt{ac}}{2abc}\)
Tiếp tục áp dụng BĐT AM-GM:
\(\sqrt{bc}+\sqrt{ac}+\sqrt{ab}\leq \frac{b+c}{2}+\frac{c+a}{2}+\frac{a+b}{2}=a+b+c\)
Do đó:
\(\frac{1}{a^2+bc}+\frac{1}{b^2+ac}+\frac{1}{c^2+ab}\leq \frac{\sqrt{ab}+\sqrt{bc}+\sqrt{ca}}{2abc}\leq \frac{a+b+c}{2abc}\) (đpcm)
Dấu "=" xảy ra khi $a=b=c$
22 tháng 2 2018
a, Từ \(\dfrac{a}{c}=\dfrac{c}{b}\Rightarrow c^2=a\cdot b\). Khi đó :
\(\dfrac{a^2+c^2}{b^2+c^2}=\dfrac{a^2+a\cdot b}{b^2+a\cdot b}=\dfrac{a\cdot\left(a+b\right)}{b\cdot\left(a+b\right)}=\dfrac{a}{b}=VP\)
⇒ĐPCM
b, Từ \(\dfrac{a}{c}=\dfrac{c}{b}\Rightarrow c^2=a\cdot b\) và áp dụng công thức \(a^2-b^2=\left(a+b\right)\cdot\left(a-b\right)\).Khi đó :
\(\dfrac{b^2-a^2}{a^2+c^2}=\dfrac{\left(b-a\right)\left(b+a\right)}{a^2+a\cdot b}=\dfrac{\left(b-a\right)\left(a+b\right)}{a\cdot\left(a+b\right)}=\dfrac{b-a}{a}=VP\)
⇒ĐPCM
30 tháng 12 2020
2: Ta có: \(\dfrac{a^2}{b+c}+\dfrac{b^2}{c+a}+\dfrac{c^2}{a+b}=\dfrac{a\left(a+b+c\right)}{b+c}+\dfrac{b\left(a+b+c\right)}{c+a}+\dfrac{c\left(a+b+c\right)}{a+b}-a-b-c=\left(a+b+c\right)\left(\dfrac{a}{b+c}+\dfrac{b}{c+a}+\dfrac{c}{a+b}\right)=a+b+c-a-b-c=0\)
30 tháng 12 2020
1: Sửa đề: Cho \(x,y,z\ne0\) và \(\dfrac{1}{x}+\dfrac{2}{y}+\dfrac{1}{z}=\dfrac{2}{2x+y+2z}\).
CM:....
Đặt 2x = x', 2z = z'.
Ta có: \(\dfrac{2}{x'}+\dfrac{2}{y}+\dfrac{2}{z'}=\dfrac{2}{x'+y+z'}\)
\(\Leftrightarrow\dfrac{1}{x'}+\dfrac{1}{y}+\dfrac{1}{z'}=\dfrac{1}{x'+y+z'}\)
\(\Leftrightarrow\dfrac{1}{x'}-\dfrac{1}{x'+y+z'}+\dfrac{1}{y}+\dfrac{1}{z'}=0\)
\(\Leftrightarrow\dfrac{y+z'}{x'\left(x'+y+z'\right)}+\dfrac{y+z'}{yz'}=0\)
\(\Leftrightarrow\dfrac{\left(y+z'\right)\left(yz'+x'^2+x'y+x'z'\right)}{x'yz'\left(x'+y+z'\right)}=0\)
\(\Leftrightarrow\dfrac{\left(x'+y\right)\left(y+z'\right)\left(z'+x'\right)}{x'yz'\left(x'+y+z'\right)}=0\Leftrightarrow\left(2x+y\right)\left(y+2z\right)\left(2z+2x\right)=0\Leftrightarrow\left(2x+y\right)\left(y+2z\right)\left(z+x\right)=0\left(đpcm\right)\)
6 tháng 3 2021
Áp dụng bđt Cô-si:
\(\dfrac{a^2}{b+c}+\dfrac{b+c}{4}\ge2\sqrt{\dfrac{a^2}{b+c}\cdot\dfrac{\left(b+c\right)}{4}}=2\sqrt{\dfrac{a^2}{4}}=a\)
Chứng minh tương tự :
\(\dfrac{b^2}{c+a}+\dfrac{c+a}{4}\ge b;\dfrac{c^2}{a+b}+\dfrac{a+b}{4}\ge c\)
\(\Rightarrow\dfrac{a^2}{b+c}+\dfrac{b^2}{c+a}+\dfrac{c^2}{a+b}+\dfrac{1}{4}\left(2a+2b+2c\right)\ge a+b+c\Rightarrow\dfrac{a^2}{b+c}+\dfrac{b^2}{c+a}+\dfrac{c^2}{a+b}\ge a+b+c-\dfrac{1}{2}\left(a+b+c\right)=\dfrac{a+b+c}{2}\) Dấu= xảy ra \(\Leftrightarrow a=b=c\)
ND
20 tháng 5 2018
Xét:
\(\dfrac{a^2}{b^2+c^2}-\dfrac{a}{b+c}=\dfrac{a\left(ab+ac-b^2-c^2\right)}{\left(b^2+c^2\right)\left(b+c\right)}=\dfrac{ab\left(a-b\right)+ac\left(a-c\right)}{\left(b^2+c^2\right)\left(b+c\right)}\left(1\right)\)
Tương tự:
\(\dfrac{b^2}{c^2+a^2}-\dfrac{b}{c+a}=\dfrac{bc\left(b-c\right)+ba\left(b-a\right)}{\left(c^2+a^2\right)\left(c+a\right)}\) (2)
\(\dfrac{c^2}{a^2+b^2}-\dfrac{c}{a+b}=\dfrac{ca\left(c-a\right)+cb\left(c-b\right)}{\left(a^2+b^2\right)\left(a+b\right)}\) (3)
Cộng từng vế (1)(2)(3) ta được:
\(\left(\dfrac{a^2}{b^2+c^2}+\dfrac{b^2}{c^2+a^2}+\dfrac{c^2}{a^2+b^2}\right)-\left(\dfrac{a}{b+c}+\dfrac{b}{c+a}+\dfrac{c}{a+b}\right)\)
\(=ab\left(a-b\right)\left[\dfrac{1}{\left(b^2+c^2\right)\left(b+c\right)}-\dfrac{1}{\left(a^2+c^2\right)\left(a+c\right)}\right]+ac\left(a-c\right)\left[\dfrac{1}{\left(b^2+c^2\right)\left(b+c\right)}-\dfrac{1}{\left(a^2+b^2\right)\left(a+b\right)}\right]+bc\left(b-c\right)\left[\dfrac{1}{\left(a^2+c^2\right)\left(a+c\right)}-\dfrac{1}{\left(a^2+b^2\right)\left(a+b\right)}\right]\) => ĐPCM
NV
Nguyễn Việt Lâm
Giáo viên
3 tháng 8 2021
\(\dfrac{a+b}{ab+c^2}=\dfrac{\left(a+b\right)^2}{\left(ab+c^2\right)\left(a+b\right)}=\dfrac{\left(a+b\right)^2}{b\left(a^2+c^2\right)+a\left(b^2+c^2\right)}\le\dfrac{a^2}{b\left(a^2+c^2\right)}+\dfrac{b^2}{a\left(b^2+c^2\right)}\)
Tương tự:
\(\dfrac{b+c}{bc+a^2}\le\dfrac{b^2}{c\left(a^2+b^2\right)}+\dfrac{c^2}{b\left(a^2+c^2\right)}\) ; \(\dfrac{c+a}{ca+b^2}\le\dfrac{c^2}{a\left(b^2+c^2\right)}+\dfrac{a^2}{c\left(a^2+b^2\right)}\)
Cộng vế:
\(VT\le\dfrac{1}{a}\left(\dfrac{b^2}{b^2+c^2}+\dfrac{c^2}{b^2+c^2}\right)+\dfrac{1}{b}\left(\dfrac{a^2}{a^2+c^2}+\dfrac{c^2}{a^2+c^2}\right)+\dfrac{1}{c}\left(\dfrac{a^2}{a^2+b^2}+\dfrac{b^2}{a^2+b^2}\right)=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\)
Ta có \(\frac{a}{c}=\frac{c}{b}\)
=> c2 = ab
a) Ta có: \(\frac{a^{2} + c^{2}}{b^{2} + c^{2}}= \frac{a^{2} + ab}{b^{2} + ab}\) = \(\frac{a\left ( a + b \right )}{b(a + b)}= \frac{a}{b}\) (đpcm)
b) Ta có: \(\frac{b^{2} - a^{2}}{a^{2} + c^{2}}= \frac{\left ( b - a \right )\left ( b + a \right )}{a^{2} + ab}= \frac{\left ( b - a \right )\left ( b + a \right )}{a\left ( b + a \right )}= \frac{b - a}{a}\) (đpcm)
b2 - a2 = (b - a)(b + a) đây là HĐT nhé có dạng: A2 - B2 = (A - B)(A + B)