LET \[h(x)=f(g(x))\] then \[h'(x)=f'(g(x)).g'(x)<0\] if \[x\ge 0\] \[\THEREFORE h(x)\] is decreasing FUNCTION \[\therefore h(x)\le h(0)\] if \[x\ge 0\] \[\therefore f(g(x))\le f(g(0))=0\] But codomain of each function is \[[0,\INFTY )\] \[\therefore f(g(x))=0\] for all \[x\ge 0\] \[\therefore f(g(x))=0\] ALSO \[g(f(x))\le g(f(0))\] [as above]