lim {[(3n^2+cn+1)/(an^2+bn)]-4n}=5
lim {[(3n^2+cn+1)-4n(an^2+bn)]/(an^2+bn)}=5
lim {[-4an^3+(3-4b)n^2+cn+1]/(an^2+bn)}=5
所以
-4a=0
3-4b=0
c/b=5
解得:
a=0
b=3/4
c=15/4
lim(n->无穷)[(3n^2+cn+1)/(an^2+bn)-4n]=5
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