A problem by Anish Shah

Level pending

Find the positive value of following expression?

6 1 + 6 1 + 6 1 + . . . . . \frac{6}{ 1 + \frac{6}{1 + \frac{6}{1 + ..... ∞} } }

Here, infinity means adding 6 1 + \frac{6}{1 + } goes on till infinity.


The answer is 2.

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8 solutions

Jubayer Nirjhor
Dec 19, 2013

Let... x = 6 1 + 6 1 + 6 1 + 6 1 + x=\dfrac{6}{1+\dfrac{6}{1+\dfrac{6}{1+\dfrac{6}{1+\dots\infty}}}}

x = 6 1 + x \Longrightarrow x=\dfrac{6}{1+x}

x ( x + 1 ) = 6 = 2 × 3 = 2 × ( 2 + 1 ) \Longrightarrow x(x+1)=6=2\times 3=2\times (2+1)

x = 2 \therefore ~~~ x=\fbox{2}

Note : The last equation is quadratic and has two solutions (the other is 3 -3 ), but the question asks for positive value so I didn't go that way... :3

You don't need to specify for the positive \textbf{positive} value. The expression is not negative because it is a positive number divided by another positive number. The negative solution you get is extraneous.

Trevor B. - 7 years, 5 months ago

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I know that. I just gave an explanation of why I ignored the negative root of the quadratic , not the original question. What you told is another explanation. :)

Jubayer Nirjhor - 7 years, 5 months ago

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It's just that your phrasing implies that the expression has multiple values and you are picking the positive one. I think it would have been better to directly say it was extraneous.

Trevor B. - 7 years, 5 months ago

good one !!!!!!!!!!1

rohit nagappa - 7 years, 5 months ago
Raj Magesh
Dec 22, 2013

Let x = 6 1 + 6 1 + . . . x = \dfrac{6}{1+\dfrac{6}{1 + ... \infty}} . Then:

x = 6 1 + x x = \dfrac{6}{1 + x}

x 2 + x 6 = 0 x^{2} + x - 6 = 0

( x + 3 ) ( x 2 ) = 0 (x + 3)(x - 2) = 0

Since only the positive value is required, x = 2 x = \boxed{2}

Budi Utomo
Dec 23, 2013

x = 6/1+x ---> x(x+1)= 6 ---> x^2 + x - 6 = 0 (x-2)(x+3) = 6 ---> x = 2 or -3(impossible). So, x is 2. Answer ; 2

Hahn Lheem
Dec 22, 2013

Let 6 1 + 6 1 + 6 1 + 6 1 + . . . = x \frac{6}{1+\frac{6}{1+\frac{6}{1+\frac{6}{1+...}}}}=x . Therefore, we can dramatically simplify this as 6 1 + x = x \frac{6}{1+x}=x . This leads to x 2 + x = 6 x^2+x=6 , or x 2 + x 6 = 0 x^2+x-6=0 . Factoring gives us ( x + 3 ) ( x 2 ) = 0 (x+3)(x-2)=0 , so the positive value of the expression is 2 \boxed{2} .

Demietra Bcruso
Dec 20, 2013

Let, Y= 6/ 1+6/(1+6/1+..infinity )
so, we can write Y =6/(1+6/Y) as Y given , solving the quadratic equation we will get

                             ( Y+3) ( Y -2) =0 .

as we have to take positive number Y = 2

Boaz Guberman
Dec 19, 2013

The key to this question lies inside the expression itself. It we let the expression equal x x :

x = 6 1 + 6 1 + 6 1 + x = \frac {6}{1+\frac {6}{1+\frac {6}{1+\ldots}}}

Then the value (that I marked with a box) is ALSO equal to x, since it's the same thing!

x = 6 1 + 6 1 + 6 1 + x = \frac {6}{1+\boxed{\frac {6}{1+\frac {6}{1+\ldots}}}}

So instead of that box, let's rewrite it as x. The reason this is important is that we now reduced the fraction that goes on until infinity, to a simple equation that we know how to solve!

x = 6 1 + x x = \frac {6}{1+x}

Let's multiply both sides by 1 + x 1+x :

x ( 1 + x ) = 6 x(1+x)=6

Now we open the parenthesis:

x + x 2 = 6 x+x^2=6

And we have a quadratic equation!

x 2 + x 6 = 0 x^2+x-6=0

( x 2 ) ( x + 3 ) = 0 (x-2)(x+3)=0

The solutions are therefore x = 2 x=2 OR x = 3 x=-3 ...

Oh wait, we have two solutions?! Remember that the question says:

Find the positive value

So the only answer left is x = 2 \boxed{x=2} .

6 1 + 6 / 1... \frac{6}{1+6/1...} = x, → 6 1 + x \frac{6}{1+x} = x

x 2 + x 6 x^2+x -6 = ( x + 3 ) (x+3) ( x 2 ) (x-2) = 0

x = 3 x=-3 or x = 2 x=2 the question asks the positive value, so x = x= 2 \boxed{2}

Lee Gao
Dec 31, 2013

Let f ( x ) = 6 1 + x f(x) = \frac{6}{1+x} and consider the sequence x , f ( x ) , f 2 ( x ) , x, f(x), f^2(x), \cdots . Suppose the sequence converges to z z for some initial x x and f f is continuous there, then continuity preserves limits, and we know that f ( z ) = z f(z) = z is a stable fixed point. Therefore, the answer comes from the solution of the quadratic equation z = 6 1 + z z = \frac{6}{1+z} , which is finitely equivalent to the quadratic z 2 + z 6 = 0 z^2 + z - 6 = 0 . The positive root of this is 2 \boxed{2} , which is our answer.

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