(2) 求导得 lim cosx = cosa
(6) t = 1/x^2
lim e^t / t = +无穷
(8) 原式 = lim (e^x -1 - x) / [ x(e^x -1) ]
= lim (e^x -1) / (e^x -1 + xe^x)
= lim e^x / [ (2+x)*e^x ]
= 1/2
(10) 原式 = lim e^( sinx lnx )
= e^ lim ( x* lnx) （sinx 等价于x）
= e^ lim lnx / (1/x)
洛必达法则
= e^ lim [ (1/x) / (-1/x^2) ]
= e^ lim(-x)
= 1