Exponential equation solving

Algebra Level 4

Find the sum of the absolute values of all possible solutions to the equation x x 2 5 x + 6 = 1. \large x^{x^{2}-5x+6} = 1.


The answer is 7.

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3 solutions

Jubayer Nirjhor
Jun 11, 2014

3 3 cases.

1. 1. x 0 x\neq 0 , x 2 5 x + 6 = 0 x^2-5x+6=0 gives x = 2 , 3 x=2,3 .

2. 2. x = 1 x=1 , x 2 5 x + 6 = 2 x^2-5x+6=2 .

3. 3. x = 1 x=-1 , x 2 5 x + 6 = 12 x^2-5x+6=12 which is even.

Answer: 2 + 3 + 1 + 1 = 7 2+3+1+1=\fbox 7 .

i did it in the same way but it appears to be more of a number theory problem

akash deep - 6 years, 12 months ago

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I agree with you.

Ashish Menon - 5 years, 5 months ago
John Mead
Jun 16, 2014

To have a b = 1 a^b=1 , we may have one of three possibilities; (1) a = 1 a=1 , (2) a = 1 a=-1 with b b even or (3) b = 0 b=0 , provided a 0 a\neq 0 .

Case (1): we have x = 1 x=1 as a solution trivially.

Case (2): we take x = 1 x=-1 and verify that the exponent is even:

( 1 ) 2 5 ( 1 ) + 6 = 1 + 5 + 6 = 12 (-1)^2-5(-1)+6=1+5+6=12 , which is even, so x = 1 x=-1 is a solution.

Case (3): we solve x 2 5 x + 6 = 0 x^2-5x+6=0 . Factoring, ( x 2 ) ( x 3 ) = 0 (x-2)(x-3)=0 , giving x = 2 x=2 and x = 3 x=3 , which are both nonzero, thus also solutions.

We answer: 1 + 1 + 2 + 3 = 1 + 1 + 2 + 3 = 7 |-1|+|1|+|2|+|3|=1+1+2+3=\boxed{7}

Eddie The Head
Jun 11, 2014

Solution Outline: \textbf{Solution Outline:}

Taking log on both sides we get ( x 2 5 x + 6 ) l n ( x ) = 0 (x^{2}-5x+6)ln(x) = 0

So either we have x 2 5 x + 6 = 0 x^{2}-5x + 6 = 0 or l n ( x ) ln(|x|) = 0.

But ln(-1) is complex.

Vinay Sipani - 7 years ago

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Evaluate the whole thing at x=1 ( ( 1 ) 2 5 ( 1 ) + 6 ) ln ( 1 ) = 12 i π \left((-1)^2-5 (-1)+6\right) \operatorname{ln}(-1)=12 i \pi and take the antilog of 12i*pi e 12 i π = 1 e^{12 i \pi}=1

so -1 is a solution too

Justin Quintos - 7 years ago

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If e 12 i π = 1 e^{12i\pi}=1

= > 12 i π = 0 => 12i\pi=0

But 12 i π 0 12i\pi\neq0

Vinay Sipani - 7 years ago

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@Vinay Sipani No, you're wrong. Its not always like that. Complex numbers work differently :)

Krishna Ar - 7 years ago

@Vinay Sipani Well, the logarithm function isn't exactly the inverse of a complex exponential.

Justin Quintos - 7 years ago

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@Justin Quintos Consider this problem

Here,-7 is not considered as it is complex but it could be shown that the inverse is not true and hence -7 can be considered.

Vinay Sipani - 7 years ago

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@Vinay Sipani –7 is indeed a solution to the equation (using complex analysis concepts).

I can provide here, if you want, a step-by-step solution as to how substituting –7 in the left side equals 16

Justin Quintos - 7 years ago

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@Justin Quintos You are right...

The answer to that question must be changed...

Vinay Sipani - 7 years ago

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