f（x）=√x,
f′（x）=1/2√x,y=f(x)在x=1/2处的切线斜率=1/2√1/2=√2/2
f（1/2）=√2/2,切线方程为y-√2/2=√2/2（x-1/2）看不懂 用设点法解下