lim[√(n^2+4n+5)-(n-1)]
=lim{[√(n^2+4n+5)-(n-1)]*[√(n^2+4n+5)+(n-1)]}/[√(n^2+4n+5)+(n-1)]【分子有理化】
=lim[(n^2+4n+5)-(n-1)^2]/[√(n^2+4n+5)+(n-1)]
=lim(n^2+4n+5-n^2+2n-1)/[√(n^2+4n+5)+(n-1)]
=lim(6n+4)/[√(n^2+4n+5)+(n-1)]
=lim[6+(4/n)]/[√(1+(4/n)+(5/n^2))+1-(1/n)]
=(6+0)/[√(1+0+0)+1-0]
=3