There Are Some Interesting Cases To Consider

Algebra Level 2

Find the sum of all integer solutions to the equation

( x 2 3 x + 1 ) x + 1 = 1. \large (x^2 - 3x + 1)^{x + 1} = 1.

3 3 9 9 0 0 1 -1

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8 solutions

There are three options for which the given equation can be satisfied:

  • (i) the exponent x + 1 = 0 x = 1 , x + 1 = 0 \Longrightarrow x = -1, for which the base x 2 3 x + 1 = 5 0 x^{2} - 3x + 1 = 5 \ne 0 ;

  • (ii) the base x 2 3 x + 1 = 1 x ( x 3 ) = 0 x = 0 , 3 x^{2} - 3x + 1 = 1 \Longrightarrow x(x - 3) = 0 \Longrightarrow x = 0, 3 ;

  • (iii) the base x 2 3 x + 1 = 1 x 2 3 x + 2 = ( x 1 ) ( x 2 ) = 0 x = 1 , 2 , x^{2} - 3x + 1 = -1 \Longrightarrow x^{2} - 3x + 2 = (x - 1)(x - 2) = 0 \Longrightarrow x = 1,2, and the exponent x + 1 x + 1 is even. This is only the case for x = 1. x = 1.

Thus the given equation is satisfied for x = 1 , 0 , 1 , 3 , x = -1, 0, 1, 3, the sum of which is 3 . \boxed{3}.

why not -1

anisha mohapatra - 5 years, 3 months ago

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1 -1 is only one of the four possible values for x x . As noted in my solution, 0 , 1 0,1 and 3 3 are also valid solutions for x x .

Brian Charlesworth - 5 years, 3 months ago

Nice idea!!

Dev Sharma - 5 years, 8 months ago

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wrong! any number to the power of 0 equals 1

Axinte Dorin - 5 years, 4 months ago

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The exponent x + 1 x + 1 equals 0 0 when x = 1 x = -1 , for which the base x 2 3 x + 1 = 5 x^{2} - 3x + 1 = 5 . So while for any non-zero number y y we have y 0 = 1 y^{0} = 1 , (depending on the context we can consider 0 0 0^{0} as being 0 , 1 0, 1 or indeterminate), in this problem when the exponent equals 0 0 the base is necessarily 5 5 .

Brian Charlesworth - 5 years, 4 months ago

Yes. Nice idea.

Ratko Pupovac - 5 years, 5 months ago

Best Logical Thinking

Arindam Kumar Paul - 5 years, 8 months ago

@Brian Charlesworth what would be the no. Of solutions if x could take complex values?

Adithya S - 4 years, 9 months ago
Bill W
Dec 20, 2015

( x 2 3 x + 1 ) ( x + 1 ) = 1 \left(x^2 - 3 \cdot x + 1 \right)^{\left(x \thinspace + \thinspace 1 \right)} = 1

ln ( x 2 3 x + 1 ) ( x + 1 ) = ln ( 1 ) \ln\left(x^2 - 3 \cdot x + 1 \right)^{\left(x \thinspace + \thinspace 1 \right)} = \ln(1)

( x + 1 ) ln ( x 2 3 x + 1 ) = 0 \left(x + 1 \right) \cdot\ln\left(x^2 - 3 \cdot x + 1 \right) = 0

ln ( x 2 3 x + 1 ) = 0 \ln\left(x^2 - 3 \cdot x + 1 \right) = 0

e ln ( x 2 3 x + 1 ) = e 0 \mathrm{e}^{ \ln\left(x^2 \thinspace - \thinspace 3 \cdot x \thinspace + \thinspace 1 \right)} = \mathrm{e}^{0}

x 2 3 x + 1 = 1 x^2 - 3 \cdot x + 1 = 1

x 2 3 x = 0 x^2 - 3 \cdot x = 0

x ( x 3 ) = 0 x\cdot\left(x - 3\right) = 0

x = 0 x 3 = 0 x = 0 \: | \: x - 3 = 0

x = 0 x = 3 x = 0 \: | \: x = 3

3 + 0 = 3 3+0 =\boxed{3}

Going from step 3 to 4 implied division by (x+1). You need to show that x does not equal negative one before doing this.

Oli Hohman - 5 years, 5 months ago

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Furthermore, the use of logarithms restricts the domain to positive values. You would need to show that there exist no solutions for which the domain can be negative. This causes the loss of the solution x=1.

Adam Davis - 4 years, 10 months ago

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In this case, the logarithm expression is ln(x^2-3x+1). Evaluated this expression at x=-1 yields ln((-1)^2-3(-1)+1) = ln(5), which is not a negative value. Therefore, x=-1 is in the domain of the function.

Oli Hohman - 4 years, 10 months ago

You miss the solutions x=1 and x=-1. Luckily for you that doesn't change the answer.

Kai Ott - 4 years, 11 months ago

x=-1 is a solution you miss when you divide by x+1

Gastón Burrull - 5 years, 4 months ago

@Bill W can u pls tell me where u are missing the solutions x= 1,-1

Adithya S - 4 years, 9 months ago

fine method

Darshan Lal - 5 years, 5 months ago

LOL! Interesting approach.

Tee Kelly - 4 years ago
Trevor Streeton
Jan 5, 2019

If n^k = 1 then n = 1^(1/k). The only real solution is when n = 1. So x^2 - 3x + 1 = 1 or x^2 -3x = 0 so x = x = 0 or x = 3 summing to 3.

Bibhor Singh
Mar 22, 2018

I simply factorized the quadratic equation and sum up the factors that were 1+2=3.

(x^2-3x+1)^ (x+1) = 1.

as 1^y =1 for any value of y,

(x^2-3x+1)^(x+1) = 1^(x+1)

=> x^2-3x+1 =1

=> x^2-3x = 0;

=> x = 0,3. :)

Sandile Khendle
Oct 10, 2016

Guys I don't understand why we equating x^2 -3x +1 to -1

-1 to an even power equals +1

David Richner - 4 years, 4 months ago
Raghav Singh
Sep 26, 2015

( x^2-3x+1 )^x+1 = 1 x^2-3x+1 = 1^(1/x+1) becose 1^anything=1 x^2-3x+1 = 1 x^2-3x = 0 x(x-3)=0 x=3,0 now putting the value of x in equation ( 9-9+1)^2= 1^2 =1 @ x=3 (0-0+1)^1= 1^1= 1 @ x=0

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