Understanding Limits: How to Solve Challenging Problems in Calculus

[mohlam12]

hello everyone
we've been doing some exercices of limits at class and there are many ones that i didn't understand... and since you don't get that chance to ask your teacher after class in Morcco, I came here for help!
here are two of the tens that i didn't undersatnd:
so to solve this limit:

lim (x-1)/(sqrt(x²+1))
x-> +infinity

you have to go from or each x Є ]-infinity,0[ U ]0, +infinity[
g(x)=(x-1)/(sqrt(x²+1))
...x(1-1/x)
g(x)=-----------------
...|x| sqrt(1+1/x²)

i just want to understand how you go from that first line to th second line !?

and also on this one:

how to go from
sqrt(x^2+x+1)-x
to:
x(sqrt(1+1/x+1/x^2)-1)) for each x Є ]0,+infinity[

i really appreciate your help, and also if there is a website that gives you the trucks to solve these kind of limits...thanks again

 

[Galileo]

It's just bringing the x outside of the brackets. Isn't is clear that x(1-1/x) is the same as (x-1) for x/=0? Just expand the brackets.
Same thing with sqrt(x^2+1). You can bring out the x^2 in (x^2+1), giving x^2(1+1/x^2) (valid for x/=0)

But you don't need it to solve the limit. Intuitively you can argue that the -1 in the numerator and the +1 in the denominator are pretty insignificant for large x, so ignoring those you get x/|x|, whose limit is clear.
You can also simply divide top and bottom of the fraction by x.