(x,y)关于原点对称的点是（-x,-y)
(x,y)满足f(x)=loga(x+1),
则（-x,-y)=(xx,yy)满足f(x)=loga(x+1),-f(xx)=loga(-xx+1)
yy=g(xx)
为yy=-loga(1-xx)