tomas

[math]x[/math] , [math]y[/math],[math]z[/math] non negative reals number and [math]x+y+z=1[/math] . prove that [math]0 \le xy+yz+zx-2xyz \le \frac{7}{27}[/math]

4 equations 4 unknowns:

but i cant do that

no these is so defeclt , i thenk it is not school homework

these is from another forum and i dont no how i can solve it thanks