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TT
17 tháng 9 2018
ta có: \(\left(a-b+c\right)^2-\left(a+b+c\right)^2\)
VT \(=\left(a-b+c\right)\left(a-b+c\right)-\left(a+b+c\right)\left(a+b+c\right)\)
\(=a^2-ab+ac-ab+b^2-bc+ac-bc+c^2-a^2-ab-ac-ab-b^2-bc-ac-c-c^2\)
= \(-4ab-4bc=VT\left(đpcm\right)\)
KB
17 tháng 9 2018
a ) \(\left(a-b+c\right)^2-\left(a+b+c\right)^2\)
\(=\left(a-b+c-a-b-c\right)\left(a-b+c+a+b+c\right)\)
\(=-2b\left(2a+2c\right)\)
\(=-4ab-4bc\left(đpcm\right)\)
b ) \(6,3-5x+x^2\)
\(=x^2-5x+\dfrac{63}{10}\)
\(=x^2-5x+\dfrac{25}{4}+\dfrac{1}{20}\)
\(=\left(x-\dfrac{5}{2}\right)^2+\dfrac{1}{20}\ge\dfrac{1}{20}>0\forall x\left(đpcm\right)\)
:D
NT
0
KK
11 tháng 3 2018
A=\(\left(a+b\right)\left(\dfrac{a}{b}+\dfrac{b}{a}\right)\)
= \(\dfrac{a}{a}+\dfrac{b}{b}+\dfrac{a}{b}+\dfrac{b}{a}\)
= \(2+\left(\dfrac{a}{b}+\dfrac{b}{a}\right)\)
Áp dụng BĐT cô si cho 2 số ta có
\(\dfrac{a}{b}+\dfrac{b}{a}\ge2\sqrt{\dfrac{a}{b}.\dfrac{b}{a}}\)
⇔\(\dfrac{a}{b}+\dfrac{b}{a}\ge2\)
⇔\(2+\left(\dfrac{a}{b}+\dfrac{b}{a}\right)\ge4\)
⇔ A ≥4
=> Min A =4
dấu "=" xảy ra khi
\(\dfrac{a}{b}=\dfrac{b}{a}\)
⇔a2=b2
⇔a=b
vậy Min A =4 khi a=b
KT
28 tháng 3 2018
\(\left(a+b\right)^2-4ab\ge0\)
\(\Leftrightarrow\)\(a^2+2ab+b^2-4ab\ge0\)
\(\Leftrightarrow\)\(a^2-2ab+b^2\ge0\)
\(\Leftrightarrow\)\(\left(a-b\right)^2\ge0\)
Dấu "=" xảy ra \(\Leftrightarrow\)\(a=b\)
\(a^2+b^2+c^2-ab-bc-ca\ge0\)
\(\Leftrightarrow\)\(2a^2+2b^2+2c^2-2ab-2bc-2ca\ge0\)
\(\Leftrightarrow\)\(\left(a^2-2ab+b^2\right)+\left(b^2-2bc+c^2\right)+\left(c^2-2ca+a^2\right)\ge0\)
\(\Leftrightarrow\)\(\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2\ge0\)
Dấu "=" xảy ra \(\Leftrightarrow\)\(a=b=c\)
17 tháng 8 2020
a) Sửa đề: \(\left(a+b\right)^2=\left(a-b\right)^2+4ab\)
Ta có: \(VP=\left(a-b\right)^2+4ab\)
\(=a^2-2ab+b^2+4ab\)
\(=a^2+2ab+b^2\)
\(=\left(a+b\right)^2=VT\)(đpcm)
b) Ta có: \(VT=\left(a-b\right)^2\)
\(=a^2-2ab+b^2\)
\(=a^2+2ab+b^2-4ab\)
\(=\left(a+b\right)^2-4ab=VP\)(đpcm)
c) Ta có: \(VP=\left(ax-by\right)^2+\left(ay+bx\right)^2\)
\(=a^2x^2-2axby+b^2y^2+a^2y^2+2aybx+b^2x^2\)
\(=a^2x^2+b^2y^2+a^2y^2+b^2x^2\)
\(=a^2\left(x^2+y^2\right)+b^2\left(x^2+y^2\right)\)
\(=\left(x^2+y^2\right)\left(a^2+b^2\right)=VT\)(đpcm)
15 tháng 8 2020
1. Ta có: \(\left(a+b\right)^2-\left(a-b\right)^2=\left(a+b+a-b\right)\left(a+b-a+b\right)\)
\(=2a.2b=4ab\)
=> đpcm
2. Ta có: \(\left(a+b\right)^2+\left(a-b\right)^2=a^2+2ab+b^2+a^2-2ab+b^2\)
\(=2a^2+2b^2=2\left(a^2+b^2\right)\)
=> đpcm
3. Ta có:\(\left(a+b\right)^2-4ab=a^2+2ab+b^2-4ab\)
\(=a^2-2ab+b^2=\left(a-b\right)^2\)
=> đpcm
4. Ta có: \(\left(a-b\right)^2+4ab=a^2-2ab+b^2+4ab\)
\(=a^2+2ab+b^2=\left(a+b\right)^2\)
PN
15 tháng 8 2020
\(a,\left(a+b\right)^2-\left(a-b\right)^2=4ab\)
\(\Leftrightarrow\left(a^2+b^2+2ab\right)-\left(a^2+b^2-2ab\right)=4ab\)
\(\Leftrightarrow a^2+b^2-a^2-b^2+2ab+2ab=4ab\)
\(\Leftrightarrow4ab=4ab\Leftrightarrow4ab-4ab=0\Leftrightarrow0=0\)(đpcm)
\(b,\left(a+b\right)^2+\left(a-b\right)^2=2\left(a^2+b^2\right)\)
\(\Leftrightarrow\left(a^2+b^2+2ab\right)+\left(a^2+b^2-2ab\right)=2\left(a^2+b^2\right)\)
\(\Leftrightarrow a^2+b^2+a^2+b^2+\left(2ab-2ab\right)=2\left(a^2+b^2\right)\)
\(\Leftrightarrow2\left(a^2+b^2\right)=2\left(a^2+b^2\right)\Leftrightarrow2\left(a^2+b^2\right)-2\left(a^2+b^2\right)=0\Leftrightarrow0=0\)(đpcm)
\(c,\left(a+b\right)^2-4ab=\left(a-b\right)^2\)
\(\Leftrightarrow\left(a^2+b^2+2ab\right)-4ab=a^2+b^2-2ab\)
\(\Leftrightarrow a^2+b^2-2ab=a^2+b^2-2ab\)
\(\Leftrightarrow\left(a-b\right)^2=\left(a-b\right)^2\Leftrightarrow\left(a-b\right)^2-\left(a-b\right)^2=0\Leftrightarrow0=0\)(đpcm)
\(d,\left(a-b\right)^2+4ab=\left(a+b\right)^2\)
\(\Leftrightarrow\left(a^2+b^2-2ab\right)+4ab=\left(a+b\right)^2\)
\(\Leftrightarrow a^2+b^2-2ab+4ab=\left(a+b\right)^2\)
\(\Leftrightarrow a^2+b^2+2ab=\left(a+b\right)^2\Leftrightarrow\left(a+b\right)^2=\left(a+b\right)^2\)
\(\Leftrightarrow\left(a+b\right)^2-\left(a+b\right)^2=0\Leftrightarrow0=0\)(đpcm)
(a + b)^2 > 4ab
<=> a^2 + 2ab + b^2 > 4ab
<=> a^2 - 2ab + b^2 > 0
<=> (a - b)^2 > 0 (đúng)
Áp dụng bđt cô - si cho 2 số không âm:
\(a+b\ge2\sqrt{ab}\)
\(\Rightarrow\left(a+b\right)^2\ge4a\)
Dấu "=" khi a = b