f'(x)=[x^2+2x-a]/(x+1)^2
f'(1)=(3-a)/4=1/2
a=1
f(1)=1
切线y=1/2*x+1/2
b=1/2
f'(x)=(x^2+2x-1)/(x+1)^2=0
x=-1+√2,x=-1-√2
(-无穷,-1-√2),减
(-1-√2,-1)增
(-1,-1+√2),增
(-1+√2,+无穷),减