Đại số 8

E

emkobit98

Tìm x^3-y^3 biết x-y=5 và x^2+y^2=15
x-y=5=>x^2+y^2-2xy=25
=>2xy=-10
=>xy=-5
x^3-y^3=(x-y)(x^2+xy+y^2)
=5.(15-5)=5.10=50
 
H

harrypham

[TEX]x-y=5 \Rightarrow (x-y)^2=25 \Rightarrow x^2-2xy+y^2=25 \Rightarrow 2xy=-10 \Rightarrow xy=-5[/TEX].
Ta có [TEX]x^3-y^3=(x-y)^3-3xy(x-y)=5^3-3 \cdot (-5) \cdot 5=200[/TEX].
 
P

pokemonno1

][TEX]x-y=5 \Rightarrow (x-y)^2=25 \Rightarrow x^2-2xy+y^2=25 \Rightarrow 2xy=-10 \Rightarrow xy=-5[/TEX].
Ta có [TEX]x^3-y^3=(x-y)^3-3xy(x-y)=5^3-3 \cdot (-5) \cdot 5=200[/TEX].[/QUOTE]
 
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