Question

[Kim Tuyền]

CMR (x+y+z)^3=x^3+y^3+3(x+y)(y+z)(z+x)

Answer 1

[Phạm Phương Anh]

Ta có:

\(\left(x+y+z\right)^3=\left[\left(x+y\right)+z\right]^3=\left(x+y\right)^3+z^3+3z\left(x+y\right)\left(x+y+z\right)=x^3+y^3+3xy\left(x+y\right)+z^3+3z\left(x+y\right)\left(x+y+z\right)=x^3+y^3+z^3+3\left(x+y\right)\left(xy+yz+xz+z^2\right)=x^3+y^3+z^3+3\left(x+y\right)\left(y+z\right)\left(x+z\right)\)

Answer 2

[Kim Tuyền]

ths