由洛比达法则(商的极限等于求导后商的极限),得
lim(x~0)a(x)x=lim(x~0)a'(x)1=lim(x~0)a'(x)=0
=>a'(0)=0
又f’(X)=1+a'(x),所以
f'(0)=1+a'(0)=1.