Trying to simplify an equation, but it's apparently wrong.

So I'm solving a limit problem, where I try to see what happens as a function approaches x = 1. I just need to simplify an equation to solve that, which is what I'm stuck on right now.

The expression is [tex]\frac{\sqrt{17-x}-4 }{\sqrt{2-x}-1 }[/tex]

I first multiply the numerator and denominator by the denominator to get that ugly square root out of there. That gets me to [tex]\frac{(\sqrt{17-x}-4) * (\sqrt{2-x}+1) }{2-x-1}[/tex]

The denominator can be simplified to -x - 1.

I then plug 1 into the equation to find the limit I'm looking for. [tex]\frac{(\sqrt{17 - 1} - 4) * (\sqrt{1}+1) }{-1 - 1}[/tex]

Doing basic math on both the numerator and denominator, I simplify this down to 0 / -2, which is just 0. However, this apparently is the wrong answer. Can anyone see if they can find if I made an error anywhere? Thanks!

Respuesta :

Step-by-step explanation:

Letting the variable x approach 1 will give us [tex]\frac{0}{0}[/tex], an indeterminate result. In such a case, we can use L'Hopital's rule where

[tex]\displaystyle \lim_{x \to c} \dfrac{f(x)}{g(x)} = \lim_{x \to c} \dfrac{f'(x)}{g'(x)}[/tex]

Let [tex]f(x) = \sqrt{17 - x} - 4[/tex] and

[tex]g(x) = \sqrt{ 2 - x} - 1[/tex]

We can see that

[tex]f'(x) = -\frac{1}{2}(17 - x)^{-\frac{1}{2}}[/tex]

and

[tex]g'(x) = -\frac{1}{2}(2 - x)^{-\frac{1}{2}}[/tex]

Applying L'Hopital's rule, we get

[tex]\displaystyle \lim_{x \to 1} \dfrac{\sqrt{17 - x} - 4}{\sqrt{ 2 - x} - 1} = \lim_{x \to 1} \dfrac{-\frac{1}{2}(17 - x)^{-\frac{1}{2}}}{-\frac{1}{2}(2 - x)^{-\frac{1}{2}}}[/tex]

[tex]\:\:\:\:\:\:\:\:= \displaystyle \lim_{x \to 1} \sqrt{\dfrac{2 - x}{17 - x}} =\sqrt{\dfrac{1}{16}} = \dfrac{1}{4}[/tex]

Therefore, as [tex]x \rightarrow 1,[/tex] the given expression approaches [tex]\frac{1}{4}[/tex].