Solve the following equation

Algebra Level 2

x + 1 x 4 + x 4 x = 1 \dfrac{\sqrt{x+1}}{x-4}+\dfrac{\sqrt{x-4}}{x}=1

Solve for x R x \in \mathbb R in the equation above.

Notation : R \mathbb R denotes the set of real numbers .

1 0 8 10

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1 solution

Tapas Mazumdar
Sep 28, 2016

I wasn't able to find an easier method for this problem than this one. This is not the correct solution but will help for the time's sake.


Observe that x > 4 x>4 for any real solutions to exist.

Now, on the RHS we have 1 1 , which of course is a rational number.

Thus, both of x + 1 x 4 \dfrac{\sqrt{x+1}}{x-4} and x 4 x \dfrac{\sqrt{x-4}}{x} must be rational.

Case 1: \underline {\text{Case 1:}} x + 1 x 4 \dfrac{\sqrt{x+1}}{x-4} is rational.

Since the options given are integers. We know that for any x Z x \in \mathbb Z , x 4 x-4 is always an integer. Also, x 4 x \neq 4 .

So, x + 1 x+1 must also be an integer for x + 1 x 4 \dfrac{\sqrt{x+1}}{x-4} to be rational.

x { 8 , 15 , 24 , 35 , 48 , } Note that x > 4 \therefore x \in \left \{8,15,24,35,48, \cdots \right \} \qquad \qquad \qquad \color{#D61F06}{\text{Note that} \ x>4}

Case 2: \underline {\text{Case 2:}} x 4 x \dfrac{\sqrt{x-4}}{x} is rational.

We know that for any x Z x \in \mathbb Z , x x is always an integer. Also, x 0 x \neq 0 .

So, x 4 x-4 must also be an integer for x 4 x \dfrac{\sqrt{x-4}}{x} to be rational.

x { 5 , 8 , 13 , 20 , 29 , 40 , } Note that x > 4 \therefore x \in \left \{5,8,13,20,29,40, \cdots \right \} \qquad \qquad \qquad \color{#D61F06}{\text{Note that} \ x>4} .

At this point, I must not say that both the solution sets have only 8 8 common in them because I wasn't able to prove this. But it is certain that whatever the next value of x x may be it will not satisfy the RHS.

But, by plugging x = 8 x=8 we see that the equation holds true. Thus, x = 8 x = \boxed{8}

I think you have it Tapas because you can show that as x increases each potential fractional component not only tends to 0 but that each fractional component for all candidate x values is less than 0.5 so that x=8 Is the only solution.

Greg Grapsas - 3 years, 4 months ago

Alternatively we know that x+1 and x-4 must be perfect squares to produce rational quotients and that the difference in these squares is exactly 5.the only perfect squares whose difference is 5 are 9 and 4 for x+1 and x-4 whose unique solution is x=8.

Greg Grapsas - 3 years, 4 months ago

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