Polynomial Power Polynomial?

Algebra Level 3

( x 3 12 x 2 x + 133 ) ( x 2 6 x 55 ) = 1 \large (x^{3}-12x^{2}-x+133)^{(x^{2}-6x-55)}=1

Find the sum of all values of x x satisfying the equation above.


The answer is 7.

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.

2 solutions

Chung Kevin
Mar 3, 2016

Let A = x 3 12 x 2 x + 133 A = x^3 - 12x^2 - x + 133 and B = x 2 6 x 55 B = x^2 -6x - 55 , then we have the equation A B = 1 A^B = 1 . So all of the solutions to this equations must satisfy at least one of the following cases:

Case one : A = 1 A = 1 .
Case two : B = 0 B = 0 but A 0 A \ne 0 .
Case three : A = 1 A = -1 , and B B is an even number.

For case one :, we have

x 3 12 x 2 x + 133 = 1 x 3 12 x 2 x + 132 = 0 x^3 - 12x^2 - x + 133 = 1\Rightarrow x^3-12x^2 - x + 132 = 0

By rational root theorem , a possible root of the cubic equation above is ± factors of 132 \pm \; \text{factors of 132} . Trial and error shows that x = 3 x = -3 is a possible solution. By performing long division of polynomials, we have

x 3 12 x 2 x + 132 = ( x + 3 ) ( x 2 15 x + 44 ) = ( x + 3 ) ( x 11 ) ( x 4 ) = 0. x^3-12x^2 - x + 132 = (x+3)(x^2 - 15x + 44) = (x+3)(x-11)(x-4) = 0 .

Thus the roots are x = 3 , 11 , 4 x=-3,11,4 . Substituting each of these values individually shows that all these 3 values are possible solutions.

For case two :, we have x 2 6 x 55 = 0 ( x + 5 ) ( x 11 ) = 0 x = 5 , 11 x^2 -6x - 55 = 0 \Rightarrow (x+5)(x-11) = 0 \Rightarrow x = -5, 11 .

Substituting each of these values individually shows that these 2 values are possible solutions as well.

For case three , we have x 3 12 x 2 x + 133 = 1 x 3 12 x 2 x + 134 = 0 x^3 - 12x^2 - x + 133 = -1 \Rightarrow x^3 - 12x^2 - x + 134 = 0 . Rational root theorem shows that there is no integer solution of x x , thus it is impossible for any of 3 roots of this cubic equation to satisfy the equation x 2 6 x 55 = some even number x^2 - 6x - 55 = \text{ some even number} .

In other words, there is no solution for case three.

Hence, gathering all the solutions from cases one to three, we have x = 3 , 5 , 4 , 11 x = -3,-5,4,11 , and their sum is 3 5 + 4 + 11 = 7 -3-5+4+11 = \boxed7 .

Perfect solution

Arsan Safeen - 5 years, 3 months ago
Chew-Seong Cheong
Feb 29, 2016

The L H S = 1 LHS = 1 when:

{ x 3 12 x 2 x + 133 = 1 . . . ( 1 ) x 2 6 x 55 = 0 . . . ( 2 ) \begin{cases} x^3 - 12x^2 -x+133 = 1 & ...(1) \\ x^2-6x-55 = 0 & ...(2) \end{cases}

( 1 ) : x 3 12 x 2 x + 132 = 0 By rational root theorem, we find that 11 is a root ( x 11 ) ( x 2 x 12 ) = 0 ( x 11 ) ( x + 3 ) ( x 4 ) = 0 x = { 3 4 11 \begin{aligned} (1): \quad x^3 - 12x^2 -x+132 & = 0 \quad \quad \small \color{#3D99F6}{\text{By rational root theorem, we find that 11 is a root}} \\ (x-11)(x^2 - x -12) & = 0 \\ (x-11)(x+3)(x-4) & = 0 \\ \Rightarrow x & = \begin{cases} -3 \\ 4 \\ 11 \end{cases} \end{aligned}

( 2 ) : x 2 6 x 55 = 0 ( x + 5 ) ( x 11 ) = 0 x = { 5 11 \begin{aligned} (2): \quad x^2-6x-55 & = 0 \\ (x+5)(x-11) & = 0 \\ \Rightarrow x & = \begin{cases} -5 \\ 11 \end{cases} \end{aligned}

Therefore, there are 4 4 distinct solutions and their sum = 5 3 + 4 + 11 = 7 = -5-3+4+11 = \boxed{7}

Nice solution

Arsan Safeen - 5 years, 3 months ago

Log in to reply

The answer will be 18 if we add 11 twice.

Atanu Ghosh - 5 years, 3 months ago

Log in to reply

You didn't mention in the QUESTION that we have to add only distinct roots.

Atanu Ghosh - 5 years, 3 months ago

Log in to reply

@Atanu Ghosh "Sum of all values of x x satisfying..." and they are 5 , 3 , 4 , 11 -5,-3,4,11 . It is not sum of all roots.

Chew-Seong Cheong - 5 years, 3 months ago

Log in to reply

@Chew-Seong Cheong Okay Sir, thankz Now i understood.

Atanu Ghosh - 5 years, 3 months ago

Log in to reply

@Atanu Ghosh Sir i have put some discussion on my profile as notes. Please check it out.

Atanu Ghosh - 5 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...