Cho x, y, z thỏa mãn:
$\frac{1}{x}+\frac{1}{y}+\frac{1}{z}= \frac{1}{x+y+z}$
a, CMR: (x+y)(y+z)(z+x) = 0
b, Tính: M = $\frac{2}{7}+ (x^{8}-y^{8})(y^{9}+z^{9})(z^{10}-x^{10})$
đk $\Leftrightarrow (x+y+z)(xy+yz+zx)=abc\Leftrightarrow (x+y)(y+z)(z+x)=0$
$neu x=-y\Rightarrow (x^8-y^8)=0$
nếu y=-z$y=-z\Rightarrow y^9+z^9=0$
neu z=-x $\Rightarrow z^(10)-x^(10)=0$
suy ra M=2:7