与当x趋近于0时sin(x)/((x+1)^2-1)=sin(x)/(x^2+2x)=1/(x+2)=1/2相当即当x→-1时，求sin（x+1）/（x^2-1）的极限为1/2
很简单嘛，当x趋近于-1时，sin（x+1）/（x^2-1）的极限等于其导数比，即sin(x+1)/(x^2-1)=(sin(x+1))'/(x^2-1)'=cos(x+1)/2x=cos(-1+1)/(2*1)=-1/2
