Question

Cho  và . Khi đó  bằng 

Answer 1

\(x-y=5\)

=> \(x^2-2xy+y^2=25\)

=>\(-2xy=25-\left(x^2+y^2\right)=25-15=10\)

=>xy=-5

\(x^3-y^3=\left(x-y\right)\left(x^2+xy+y^2\right)=5\cdot\left(15-5\right)=5\cdot10=50\)

Answer 2

Ta có :

\(x-y=15\)

\(\Rightarrow x^2+y^2-2xy=25\)

\(\Rightarrow15-2xy=25\)

\(\Rightarrow xy=-5\)

\(\Rightarrow-xy=5\)

Mặt khác : \(x^3-y^3\)

\(=\left(x-y\right)\left(x^2+y^2+xy\right)\)

\(=5\left(15+5\right)\)

\(=15.20\)

\(=100\)