Hi everybody,

I don't know how to show that the following sets aren't linear subspaces:

$\displaystyle F_1$={$\displaystyle (x;y;z)/x+y+z=1$}

$\displaystyle F_2$={$\displaystyle (x;y;z)/xy=0$}

$\displaystyle F_3$={$\displaystyle (x;y;z)/x^2=z^2$}

...

Can you help me please????