A Golden Substitution

Algebra Level 1

Given that x 2 = x + 1 x^2 = x +1 , find the value of 1 x + 1 x + 1 \frac{1}{x} + \frac{1}{x+1}


The answer is 1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

16 solutions

Michael Ng
Mar 30, 2015

What I like about this is that you don't have to solve the quadratic! In fact, notice that by substitution: 1 x + 1 = 1 x 2 \frac{1}{x+1} = \frac{1}{x^2} due to the given fact.

Now simplify: 1 x + 1 x + 1 = 1 x + 1 x 2 = x + 1 x 2 \frac{1}{x} + \frac{1}{x+1} \\ = \frac{1}{x} + \frac{1}{x^2} \\ = \frac{x+1}{x^2}

But remember that x 2 = x + 1 x^2 = x+1 , so x + 1 x 2 = x 2 x 2 = 1 \frac{x+1}{x^2} = \frac{x^2}{x^2} = \boxed{1}

Moderator note:

Nice. An alternative approach is 1 x + 1 x + 1 = x x 2 + 1 x + 1 = x x + 1 + 1 x + 1 = x + 1 x + 1 = 1 \dfrac 1 x + \dfrac 1 {x+1} = \dfrac x {x^2} + \dfrac 1 {x+1} = \dfrac x {x+1} + \dfrac {1}{x+1} = \dfrac{x+1}{x+1} = 1 .

At the point at which you get 1 x + 1 x 2 \frac{1}{x} + \frac{1}{x^2} , an even faster way is to notice that dividing x 2 = x + 1 x^2 = x+1 by x 2 x^2 gives 1 = 1 x + 1 x 2 1 = \frac{1}{x} + \frac{1}{x^2} which leads directly to the solution.

Michael Ng - 6 years, 2 months ago

Doesn't compute. 1 squared=1 That equals 1+1=2. 1/1+1/2=1 1/2. How do you figure 1=2=1 1/2? Yes, one would have been my answer glancing at it but it doesn't work. Couldn't figure another answer that did.

Art Boggs - 6 years, 2 months ago

Log in to reply

I am not solving for x x but I am finding the value of the expression given instead. In fact there are two possible values of x x ; 1 ± 5 2 \frac{1\pm\sqrt{5}}{2} .

Michael Ng - 6 years, 2 months ago
Chew-Seong Cheong
Mar 30, 2015

It is given that: x 2 = x + 1 x^2 = x+1 . Dividing throughout by x 2 x^2 , we have 1 = 1 x + 1 x 2 1=\dfrac{1}{x}+\dfrac {1}{x^2} . Since x 2 = x + 1 x^2 = x+1 , then 1 x + 1 x + 1 = 1 x + 1 x 2 = 1 \dfrac{1}{x}+\dfrac {1}{x+1} = \dfrac{1}{x}+\dfrac {1}{x^2} = \boxed{1} .

Caleb Townsend
Mar 30, 2015

x 2 = x + 1 1 x + 1 x + 1 = 1 x + 1 x 2 = x + 1 x 2 = x 2 x 2 = 1 x^2 = x + 1 \\ \Rightarrow \frac{1}{x} + \frac{1}{x+1} = \frac{1}{x} + \frac{1}{x^2} \\ = \frac{x+1}{x^2} \\ = \frac{x^2}{x^2} \\ = \boxed{1} This doesn't account for the possibilities x = 0 x=0 or x = 1 , x=-1, so we need to check if they satisfy the original equation: 0 2 0 + 1 ( 1 ) 2 1 + 1 0^2 \neq 0 + 1 \\ (-1)^2 \neq -1 + 1

Shashi Kamal
Mar 31, 2015

Just look at the question you will get the answer.

I didnt get it by looking -_-

Mehul Arora - 6 years, 2 months ago
Rwit Panda
Mar 31, 2015

When we proceed for (1÷x) + [1÷(x+1)], we get (2x + 1)÷(x^2 + x). Substituting numerator x + x + 1, we get x + x^2 {from given equation}. Thus numerator and denominator cancel each other out and we obtain result as 1.

Tarun Kansliwal
Mar 31, 2015

Just simply take the LCM, 1/x + 1/(x+1) = (x+1+x)/x(x+1) = ( x^2 +x )/ x.x^2 = x(x +1)/ x.x^2 = 1

Aman Real
Mar 31, 2015

x²=x+1.now,x²-x=1.now,x(x-1)=1,now,x-1=1/x,now substituting in 1/x +1/x+1. We do ,(x-1)+1/(x+1). Solving it we get 1

Cleres Cupertino
Aug 20, 2015

x 2 = x + 1 x = x + 1 x 1 x = x x + 1 1 x = x + 1 1 x + 1 1 x = 1 1 x + 1 x^2=x+1 \Rightarrow x=\frac{x+1}{x} \Rightarrow \frac{1}{x}=\frac{x}{x+1} \Rightarrow \frac{1}{x}=\frac{x+1-1}{x+1} \Rightarrow \frac{1}{x}=1-\frac{1}{x+1}

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

x^{2}=x+1 -> x^{2}-x=1 -> x(x-1)= thus, x=1

Ngan Tran
Apr 18, 2015

Here is what I did

Gamal Sultan
Apr 15, 2015

1/x + 1/(x + 1) = 1/x + 1/x^2 = (x + 1)/x^2 = 1

Ademar García
Apr 10, 2015

This is the same as 1/x + 1/x^2 or x/x^2 + 1/x^2 or (x+1)/x^2 or x^2 over x^2 which is 1.

Robert Ricafort
Apr 10, 2015

Well first I modified the first equation and factored it

I got these results. I substituted -1 to x in the expression, however one of the denominators would be 0 making it undefined. So I substituted 2 to the expression and got this:

Since the answer should be an integer, I rounded it up and answered 1 rather than 0.83333.....

That's not a correct factorisation. ( x 2 ) ( x + 1 ) = x 2 x 2 (x-2)(x+1) = x^2 - x - 2

Paul Smith - 6 years, 1 month ago

The answer is 1.

If x^2 = x + 1, then x^2 + x = x + 1 + x <=> x^2 + x = 2x + 1

So:

1/x + 1/(x + 1) = (x + 1 + x)/(x^2 + x) = (2x + 1)/(x^2 + x) =

= (2x + 1)/(2x + 1) = 1

Hobart Pao
Apr 1, 2015

What i did is to find like denominators, first. This obtains 2 x + 1 x 2 + x \displaystyle\frac{2x+1}{x^{2}+x} If you add x to both sides of the given, you get that x 2 + x = 2 x + 1 x^{2} + x = 2x + 1 Thus, the answer is 1.

Jason Hughes
Mar 31, 2015

Since x 2 = x + 1 , x 2 + x = 2 x + 1 x^2=x+1, x^2+x=2x+1 .

1 x + 1 x + 1 = 2 x + 1 x 2 + x = 2 x + 1 2 x + 1 = 1. \dfrac{1}{x}+\dfrac{1}{x+1} = \dfrac{2x+1}{x^2+x}=\dfrac{2x+1}{2x+1}=1.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...