x趋近于正无穷 arctan(x-cosxlnx) =arctan【x(1-cosx*lnx/x )】 =arctanx =π/2
因为x趋近于正无穷时 ,由“无穷小*有界函数=无穷小“ 得cosx/x=0,所以x(1-cosx*lnx/x =x