用无穷小替换 lim x趋向无穷 (2x^2 - 1)/(X^3+x-1)的整体乘（arctanx+cosx+1）

lim(x->∞) (2x^2 - 1) /(x^3+x-1) = 0
x->+∞,arctanx ->π/2 ; x->-∞,arctanx -> -π/2
当x->∞时,（arctanx+cosx+1）为有界函数,
无穷小与有界函数的乘积还是无穷小,于是
lim(x->∞) 【(2x^2 - 1) /(x^3+x-1) *（arctanx+cosx+1）】= 0