lim(x→∞） (x^2-1)/2x
order of x^2-1 =2
order of 2x =1
lim(x→∞） (x^2-1)/2x = +∞为什么 x^2-1 =2 2x =1order of the polynomial = 多项式的阶order of x^2-1 = 2( x^2 =2 )order of 2x =1 ( x = x^1 = 1 )