f'(x)=e^x-a f'(ln2)=2-a=1 so a=1
e^x-x-1>=mx^2 for all x>=0 等价于 g(x)=e^x-1-x-mx^2>=0 for all x>=0
since g(0)=0,g'(x)=e^x-1-2mx
易证 当m