Cho x,y,x>0 ; x+y+z=1.a)CM: $\frac{yz}{1-x}+\frac{xz}{1-y}+\frac{yx}{1-z}\leqslant \frac{1}{2}$
b) $(\frac{1}{1+x})(\frac{1}{1+y})(\frac{1}{z+1})\geqslant 64$
c) $\sqrt{\frac{yz}{x+yz}}+\sqrt{\frac{zx}{zx+y}}+\sqrt{\frac{xy}{xy+z}}\leq \frac{3}{2}$
d) $\frac{x^2}{y+z}+\frac{y^2}{x+z}+\frac{z^2}{x+y} \geqslant \frac{1}{2}$