右极限 limf(x) = limln(1+x^2)/(1-cosx)= limx^2/(x^2/2) = 2,左极限 limf(x) = lim2sinx/(e^x-1)+a= lim2x/x + a = 2+a在 x = 0 极限存在， 2+a = 2， a = 0.
