高等數學極限的問題？

01-15

這步怎麼等於2

$eqalign{ mathop {lim }limits_{n o infty } frac{{4sqrt n }}{{sqrt {n + 3sqrt n } + sqrt {n - sqrt n } }} cr sqrt n = u,n = {u^2}, cr = mathop {lim }limits_{u o infty } frac{{4u}}{{sqrt {{u^2} + 3u} + sqrt {{u^2} - u} }} cr = mathop {lim }limits_{u o infty } frac{{4frac{u}{u}}}{{frac{{sqrt {{u^2} + 3u} + sqrt {{u^2} - u} }}{u}}} cr = mathop {lim }limits_{u o infty } frac{4}{{sqrt {1 + 3frac{1}{u}} + sqrt {1 - frac{1}{u}} }} cr = frac{4}{{sqrt {1 + 0} + sqrt {1 - 0} }} cr = 2 cr}$