/sci/ - Science & Math

If there is a limit going towards some value [math]a^2[/math] such that [math]a>0[/math] is in the argument's denominator [math]\sqrt{x}+a[/math], why do we ignore the negative root of [math]a^2[/math] here and just proceed to let [math]x=a^2[/math] and get rid of the limit?