x+y=f(y)两边同时求导得到:1+y'=f'(y)*y'即:1+f'(x)=f'(y)f'(x)因为f'(1)=1所以:1+1=f'(y)*1所以:f'(y)=2.则:1+y'=2y'即y'=1.所以y''=0.