lim(n->无穷)[(3n^2+cn+1)/(an^2+bn)-4n]=5
问题描述：
lim(n->无穷)[(3n^2+cn+1)/(an^2+bn)-4n]=5
求常数a、b、c
答
lim {[(3n^2+cn+1)/(an^2+bn)]-4n}=5lim {[(3n^2+cn+1)-4n(an^2+bn)]/(an^2+bn)}=5lim {[-4an^3+(3-4b)n^2+cn+1]/(an^2+bn)}=5所以-4a=03-4b=0c/b=5解得:a=0 b=3/4 c=15/4