✔ 最佳答案
lim(x->∞)_[√(9x^2+5x+4)-ax-b]=0,求 a、b
Sol
A=lim(x->∞)_[√(9x^2+5x+4)-ax-b]
=lim(x->∞)_[√(9x^2+5x+4)-(ax+b)]*[√(9x^2+5x+4)+(ax+b)]
/[√(9x^2+5x+4)+(ax+b)]
=lim(x->∞)_[(9x^2+5x+4)-(ax+b)^2]/[√(9x^2+5x+4)+(ax+b)]
=lim(x->∞)_[(9x^2+5x+4)-(a^2x^2+2abx+b^2)]/[√(9x^2+5x+4)+(ax+b)]
=lim(x->∞)_[(9-a^2)x^2+(5-2ab)x+4-b^2)]/[√(9x^2+5x+4)+(ax+b)]
So
9-a^2=0
a=3 or a=-3(不合)
5-2ab=0
5-6b=0
b=5/6