Polynomial Exponent

Algebra Level 4

( x 2 7 x + 11 ) x 2 6 x 7 = 1 (x^2-7x+11)^{x^2-6x-7}=1

Find the sum of the values of x x that satisfy the above equation.

This problem is not original.
20 16 13 7 None of these answers

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3 solutions

Discussions for this problem are now closed

Trevor B.
Jan 31, 2015

There are three cases to check when solving this problem: when the base is equal to 1 , 1, when the exponent is equal to 0 0 and the base is not equal to 0 , 0, and when the base is equal to 1 -1 and the exponent is divisible by 2. 2.


Case 1: \textbf{Case 1:} x 2 7 x + 11 = 1 x^2-7x+11=1

x 2 7 x + 11 = 1 x^2-7x+11=1

x 2 7 x + 10 = 0 \Rightarrow x^2-7x+10=0

( x 2 ) ( x 5 ) = 0 \Rightarrow(x-2)(x-5)=0

This shows that x = 2 x=2 and x = 5 x=5 are solutions.

Case 2: \textbf{Case 2:} x 2 6 x 7 = 0 x^2-6x-7=0

x 2 6 x 7 = 0 x^2-6x-7=0

( x + 1 ) ( x 7 ) = 0 \Rightarrow(x+1)(x-7)=0

Checking x = 1 x=-1 and x = 7 x=7 in the base, we find that neither of those cause the base to be equal to 0 , 0, so x = 1 x=-1 and x = 7 x=7 can be accepted as solutions.

Case 3: \textbf{Case 3:} x 2 7 x + 11 = 1 , x 1 mod 2 x^2-7x+11=-1, x\equiv1\text{ mod }2

x 2 7 x + 11 = 1 x^2-7x+11=-1

x 2 7 x + 12 = 0 \Rightarrow x^2-7x+12=0

( x 3 ) ( x 4 ) = 0 \Rightarrow(x-3)(x-4)=0

Checking x = 3 x=3 and x = 4 x=4 in the exponent, we find that x = 3 x=3 causes the exponent to be divisible by 2 2 but x = 4 x=4 does not. Thus, we can only accept x = 3 x=3 as a solution.


Our solutions are x { 1 , 2 , 3 , 5 , 7 } . x\in\{-1,2,3,5,7\}. The sum of the solutions is 16 \boxed{16}

Justin Wong I got this wrong today because I forgot the base = 1 =-1 case. Did you?

Trevor B. - 6 years, 4 months ago

Hey same here!!

Utkarsh Bansal - 6 years, 4 months ago

same here buddy..

abhijeet raj - 6 years, 4 months ago

yea i got that wrong becuz of that

Alex Segesta - 6 years, 4 months ago

I didn't notice that this is Level 5. Thought too much about others. Initially thought that the figures are total number of roots. Too happy to find 4 of 13.

Lu Chee Ket - 6 years, 4 months ago

Yeah, the same thing.

Dhruv G - 6 years, 4 months ago

Ohh !!same here I too missed the 3rd case ...

tanmay goyal - 6 years, 4 months ago

Same I missed the 3rd.

Anan Quan - 6 years, 4 months ago

in case 1, we could use vieta's to determine the sum

Aareyan Manzoor - 6 years, 4 months ago

Nice. I thought this would be a lot harder.

Milly Choochoo - 6 years, 4 months ago

Shouldn't x 1 ( m o d 2 ) x\equiv 1 \pmod{2}

Trevor Arashiro - 6 years, 4 months ago

the question should mention 'real' because x 2 7 x + 11 = e x p ( 2 k π n i ) x^2-7x+11=exp(\dfrac{2k\pi}{n}i) x 2 6 x 7 = z n x^2-6x-7=zn for some integer k,z and n, than the expression would be 1

Aareyan Manzoor - 6 years, 3 months ago

Overrated!

Kartik Sharma - 6 years, 4 months ago
Vikram Waradpande
Jan 31, 2015

The condition of the problem is satisfied if one of the following cases hold true: Case 1 \text{Case 1} : x 2 7 x + 11 = 1 x^2 -7x+11 = 1 and the exponent can be any number. This yields x = 5 , 2 x=5,2 Case 2 \text{Case 2} : x 2 7 x + 11 = 1 x^2-7x+11 = -1 and the exponent must be even. This yields x = 3 x= 3

Case 3 \text{Case 3} : x 2 6 x 7 = 0 x^2-6x-7=0 This yields x = 7 , 1 x= 7, -1

So the sum of all values of x x is 5 + 2 + 3 + 7 1 = 16 5+2+3+7-1 = \boxed{16}

Paola Ramírez
Feb 1, 2015

Case 1: x 2 6 x 7 = 0 x^2-6x-7=0

x 2 6 x 7 = 0 x^2-6x-7=0

( x 7 ) ( x + 1 ) = 0 (x-7)(x+1)=0

x = 7 , 1 x=7,-1

Case 2: x 2 7 x + 11 = 1 x^2-7x+11=1

x 2 7 x + 11 = 1 x^2-7x+11=1

x 2 7 x + 10 = 0 x^2-7x+10=0

( x 5 ) ( x 2 ) = 0 (x-5)(x-2)=0

x = 5 , 2 x=5,2

Case 3: x 2 7 x + 11 = 1 x^2-7x+11=1 and x 2 6 x 7 = 2 k x^2-6x-7=2k

As x 2 6 x 7 = ( x 7 ) ( x + 1 ) , ( x 7 ) x^2-6x-7=(x-7)(x+1), (x-7) and ( x + 1 ) (x+1) have the same parity \therefore x 2 6 x 7 = 2 k x^2-6x-7=2k only for odd x x

x 2 7 x + 11 = 1 x^2-7x+11=-1

x 2 7 x + 12 = 0 x^2-7x+12=0

( x 4 ) ( x 3 ) = 0 (x-4)(x-3)=0

Then odd x x and solution of x 2 7 x + 11 = 1 x^2-7x+11=-1 is x = 3 x=3

The sum of all values for x x is 7 1 + 5 + 2 + 3 = 16 7-1+5+2+3=\boxed{16}

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