# Finding the values of the real constants such that the limit exists

Find the values of the real constants $c$ and $d$ such that

$$\lim_{x\to 0}\frac{\sqrt{c+dx}-\sqrt{3}}{x}=\sqrt{3}$$

I really have no clue how to even get started.

• Been wondering if I should do a mathematics major if I can't even comprehend a question this simple...
– kingboonz
Sep 13 '12 at 9:26
• Start with Series[Sqrt[c + d x], {x, 0, 1}]/x; how to make this finite and equal to Sqrt in the limit ?
– b.gatessucks
Sep 13 '12 at 9:31
• This is more of a math question than a Mathematica one, so I'm migrating this to math.SE. Sep 13 '12 at 12:30
• @J.M. I don't agree, this question is mathematically very simple, however Mathematica hardly tackles this problem. So I find this a valuable Mathematica question and upvoted it. Sep 13 '12 at 12:37
• @Artes, considering the "I realised I posted on the wrong forum" comment... Sep 13 '12 at 12:39

I don't know a way Mathematica is going to solve that directly for you, but it can help you understand what happens. First, let's ask it what the general expression for the limit is:

Limit[(Sqrt[c + d*x] - Sqrt)/x, x -> 0]


DirectedInfinity[-Sqrt + Sqrt[c]]


So, this limit is an infinity, and has the same sign as $\sqrt c-\sqrt 3$. If you want the limit to be finite, you have only one special case you can try: what happens if $\sqrt c-\sqrt 3=0$, i.e. if $c=3$?

Limit[(Sqrt[3 + d*x] - Sqrt)/x, x -> 0]


The results is:

d/(2 Sqrt)


which is finite, so all is well. Now, you need this to be equal to $\sqrt 3$, which you can sure solve yourself, but just to be thorough, let's ask Mathematica to do it:

Solve[Limit[(Sqrt[3 + d*x] - Sqrt)/x, x -> 0] == Sqrt, d]


Conclusion: no silver bullet, but definitely a way to help you understand this equation if your math skills fail you!

• Thanks F'x. You have been an amazing help with the pictures in my questions. I realised I posted on the wrong forum w/e. Thanks anyway!!!
– kingboonz
Sep 13 '12 at 9:58

Method I

Since we have a fraction going to a non-vanishing value given its denominator is going to 0 we have to assume that its numerator also tends to 0, therefore we should solve :

Reduce[Limit[Numerator[(Sqrt[c + d*x] - Sqrt)/x], x -> 0] == 0, {c, d}]

c == 3


We could use Solve as well with the option : InverseFunctions -> True, now taking the value into the system we have :

Reduce[ c == 3 && Limit[(Sqrt[3 + d*x] - Sqrt)/x, x -> 0] == Sqrt, {c, d}]

c == 3 && d == 6


Method II

Another way to tackle the problem is to expand the expression in a power series around x == 0 :

Normal @ Series[ (Sqrt[c + d*x] - Sqrt)/x, {x, 0, 3}] Now you can find limits of every term separately :

Limit[#, x -> 0] & /@ List @@ Normal@Series[(Sqrt[c + d*x] - Sqrt)/x, {x, 0, 5}]

{d/(2 Sqrt[c]), (-Sqrt + Sqrt[c]) Infinity, 0, 0, 0, 0, 0}


The only problem comes from the second term, however we can get rid of it by assuming a priori its value by setting c == 3 or simply solving this system :

Solve[ {( -Sqrt + Sqrt[c])/x == 0,  d/(2 Sqrt[c]) == Sqrt}, {c, d},
InverseFunctions -> True]

 {{c -> 3, d -> 6}}


$$\lim_{x\to 0}\frac{\sqrt{c+dx}-\sqrt{3}}{x}=\sqrt{3}$$

Since the denominator goes to $0$, the limit cannot exist unless the numerator also goes to $0$. The numerator is $\sqrt{c+dx}-\sqrt{3}$, so that would have to go to $0$ as $x$ goes to $0$. But it goes to $\sqrt{c+d\cdot0} - \sqrt{3}$. Hence $c+d\cdot0$ must be $3$. That tells you $c$ is $3$, and you've got $$\lim_{x\to 0}\frac{\sqrt{3+dx}-\sqrt{3}}{x}=\sqrt{3}.$$

Now rationalize the numerator: $$\frac{\sqrt{3+dx}-\sqrt{3}}{x} = \frac{\left(\sqrt{3+dx}-\sqrt{3}\right)\left(\sqrt{3+dx}+\sqrt{3}\right)}{x\left(\sqrt{3+dx}+\sqrt{3}\right)}= \frac{dx}{x\left(\sqrt{3+dx}+\sqrt{3}\right)}.$$ The $x$s cancel and we get $$\frac{d}{\sqrt{3+dx}+\sqrt{3}}.$$ The limit of that as $x\to0$ is $d/(2\sqrt{3})$. So you want $d/(2\sqrt{3}) = \sqrt{3}$. Multiply both sides by $2\sqrt{3}$ and you get $d=6$.