Cho \(a+b+c+d=0\) CMR: \(a^3+b^3+c^3+d^3=3\left(c+d\right)\left(ab-cd\right)\)
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KB
15 tháng 9 2018
2 ) b )
\(a+b+c+d=0\)
\(\Leftrightarrow a+b=-\left(c+d\right)\)
\(\Leftrightarrow\left(a+b\right)^3=-\left(c+d\right)^3\)
\(\Leftrightarrow a^3+b^3+3a^2b+3b^2a=-c^3-3c^2d-3d^2c-d^3\)
\(\Leftrightarrow a^3+b^3+3a^2b+3b^2a+c^3+3c^2d+3d^2c+d^3=0\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=-3a^2b-3b^2a-3c^2d-3d^2c\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=-3ab\left(a+b\right)-3cd\left(c+d\right)\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=3ab\left(c+d\right)-3cd\left(c+d\right)\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=3\left(ab-cd\right)\left(c+d\right)\) \(\left(đpcm\right)\)
TT
1
5 tháng 7 2023
\(a+b+c+d=0\Rightarrow a+b=-\left(c+d\right)\)
\(\Rightarrow\left(a+b\right)^3=-\left(c+d\right)^3\)
\(\Rightarrow\left(a+b\right)^3+\left(c+d\right)^3=0\)
\(\Rightarrow a^3+b^3+3ab\left(a+b\right)+c^3+d^3+3cd\left(c+d\right)=0\)
\(\Rightarrow a^3+b^3+c^3+d^3=-3ab\left(a+b\right)-3cd\left(c+d\right)\)
\(\Rightarrow a^3+b^3+c^3+d^3=3ab\left(c+d\right)-3cd\left(c+d\right)\) (do \(a+b=-\left(c+d\right)\)
\(\Rightarrow a^3+b^3+c^3+d^3=3\left(ab-cd\right)\left(c+d\right)\)
NN
1
2 tháng 7 2017
Giải:
Từ \(a+b+c+d=0\Leftrightarrow a+c=-\left(b+d\right)\)
\(\Leftrightarrow\left(a+c\right)^3=-\left(b+d\right)^3\)
\(\Leftrightarrow a^3+c^3+3ac\left(a+c\right)=-\left[b^3+d^3+3bd\left(b+d\right)\right]\)
\(\Leftrightarrow VT=a^3+b^3+c^3+d^3=-3bd\left(b+d\right)-3ac\left(a+c\right)\)
\(=-3bd\left(b+d\right)+3ac\left(b+d\right)=3\left(ac-bd\right)\left(b+d\right)=VP\) (Đpcm)
CV
3
28 tháng 11 2016
ta có
a+b+c+d=0
=> b+c=-(a+d) => (b+c)3=-(a+d)3
=> b3+c3+3bc(b+c)= -[a3+d3+3ad(a+d)]
=> a3+b3+c3+d3=-3ad(a+d)-3bc(b+c)= 3ad(b+c)-3bc(b+c)
=3(b+c)(ad-bc)
DT
1
31 tháng 3 2020
Theo đề, a+b+c+d=0
\(\Rightarrow a+b=-\left(c+d\right)\)
Ta có: \(VT=\left(a+b\right)\left(a^2-ab+b^2\right)+\left(c+d\right)\left(c^2-cd+d^2\right)\)
\(\Leftrightarrow VT=\left(c+d)\left(c^2-cd+d^2-a^2+ab-b^2\right)\right)\)
Để có ĐPCM ta xét hiệu: \(c^2-cd+d^2-a^2+ab-b^2-3\left(ab+cd\right)=c^2-4cd+d^2-a^2-2ab-b^2=c^2-4cd+d^2-\left(a+b\right)^2=c^2-4cd+d^2-\left(c+d\right)^2=-6cd\)
S nó ko = 0 ta:::xem lại đề..Hay mk lm sai j đó
L
2
DH
22 tháng 7 2017
Ta có:
\(a+b+c+d=0\Rightarrow a+b=-\left(c+d\right)\)
Do đó: \(\left(a+b\right)^3=-\left(c+d\right)^3\)
\(\Rightarrow a^3+3a^2b+3ab^2+b^3=-c^3-3c^2d-3cd^2-d^2\)
\(\Rightarrow a^3+3ab\left(a+b\right)+b^3=-c^3-3cd\left(c+d\right)-d^2\)
\(\Rightarrow a^3+b^3+c^3+d^3=-3cd\left(c+d\right)-3ab\left(a+b\right)\)
Vì \(a+b=-\left(c+d\right)\) nên
\(\Rightarrow a^3+b^3+c^3+d^3=3cd\left(a+b\right)-3ab\left(a+b\right)\)
\(\Rightarrow a^3+b^3+c^3+d^3=3\left(a+b\right)\left(cd-ab\right)\)
\(\Rightarrow a^3+b^3+c^3+d^3-3\left(a+b\right)\left(cd-ab\right)=0\)
Chúc bạn học tốt!!!
PP
22 tháng 7 2017
<br class="Apple-interchange-newline"><div id="inner-editor"></div>⇔a+c = -( b+ d)
⇔(a+c)3 = - ( b+d)3
⇔a3 + c3 + 3ac.(a+c) = - [ b3 + d3 + 3bd( b+d) ]
⇔a3 + b3 + c3 + d3 = -3bd(b+d) - 3ac(a+c)
⇔a3+b3+c3+d3= -3bd( b+d) + 3ac( b+d)
⇔a3+b3+c3+d3=3.(ac-bd)(d+b)
KB
18 tháng 9 2018
a ) Ta có : \(a+b+c=0\)
\(\Leftrightarrow a^2+b^2+c^2+2\left(ab+ac+bc\right)=0\)
\(\Leftrightarrow a^2+b^2+c^2=-2\left(ab+ac+bc\right)\)
\(\Leftrightarrow\left(a^2+b^2+c^2\right)^2=4\left(ab+ac+bc\right)^2\)
\(\Leftrightarrow a^4+b^4+c^4+2a^2b^2+2b^2c^2+2a^2c^2=4\left(a^2b^2+b^2c^2+c^2a^2+2ab^2c+2a^2bc+2c^2ab\right)\)
\(\Leftrightarrow a^4+b^4+c^4=2\left(a^2b^2+b^2c^2+c^2a^2\right)+8abc\left(a+b+c\right)\)
\(\Leftrightarrow a^4+b^4+c^4=2\left(a^2b^2+b^2c^2+a^2c^2\right)+8abc.0\)
\(\Leftrightarrow a^4+b^4+c^4=2\left(a^2b^2+b^2c^2+a^2c^2\right)\)
Lại có : \(\dfrac{\left(a^2+b^2+c^2\right)^2}{2}=\dfrac{a^4+b^4+c^4+2\left(a^2b^2+b^2c^2+c^2a^2\right)}{2}\)
\(=\dfrac{a^4+b^4+c^4+a^4+b^4+c^4}{2}=\dfrac{2\left(a^4+b^4+c^4\right)}{2}\)
\(=a^4+b^4+c^4\left(đpcm\right)\)
KB
18 tháng 9 2018
b ) \(a+b+c+d=0\)
\(\Leftrightarrow a+b=-\left(c+d\right)\)
\(\Leftrightarrow\left(a+b\right)^3=-\left(c+d\right)^3\)
\(\Leftrightarrow\left(a+b\right)^3+\left(c+d\right)^3=0\)
\(\Leftrightarrow a^3+b^3+c^3+d^3+3a^2b+3b^2a+3c^2d+3d^2c=0\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=-3a^2b-3b^2a-3c^2d-3d^2c\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=3\left(-a^2b-b^2a-c^2d-d^2c\right)\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=3\left[-ab\left(a+b\right)-cd\left(c+d\right)\right]\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=3\left[ab\left(c+d\right)-cd\left(c+d\right)\right]\)
\(\Leftrightarrow a^3+b^3+c^3+d^3=3\left(ab-cd\right)\left(c+d\right)\left(đpcm\right)\)
a+b+c+d=0
nên a+b=-(c+d)
\(a^3+b^3+c^3+d^3\)
\(=\left(a+b\right)^3-3ab\left(a+b\right)+\left(c+d\right)^3-3cd\left(c+d\right)\)
\(=\left[-\left(c+d\right)\right]^3-3ab\cdot\left[-\left(c+d\right)\right]+\left(c+d\right)^3-3cd\left(c+d\right)\)
\(=3ab\left(c+d\right)-3cd\left(c+d\right)\)
\(=3\left(c+d\right)\left(ab-cd\right)\)