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Algebra Level 5

Consider the following equation in ' x x ' : log 2 x + 3 ( 6 x 2 + 23 x + 21 ) = 4 log 3 x + 7 ( 4 x 2 + 12 x + 9 ) \log_{2x+3} (6x^2 + 23x + 21) = 4 - \log_{3x+7} (4x^2 + 12x + 9)

The sum of possible values of ' x x ' satisfying the above equation is of the form a b \dfrac{-a}{b} ; where a a and b b are co-prime integers.

Find a + b \mathbb{ a + b }


The answer is 5.

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3 solutions

Let p = 2 x + 3 p = 2x + 3 and q = 3 x + 7 q = 3x + 7 . Then the given equation can be written as

log p ( p q ) = 4 log q ( p 2 ) \log_{p}(p * q) = 4 - \log_{q}(p^{2}) \Longrightarrow 1 + log p ( q ) = 4 2 log q ( p ) 1 + \log_{p}(q) = 4 - 2 * \log_{q}(p) .

Now by the change-of-base rule we have that log p ( q ) = 1 log q ( p ) \log_{p}(q) = \dfrac{1}{\log_{q}(p)} .

So letting z = log q ( p ) z = \log_{q}(p) , and assuming that z 0 z \ne{0} , i.e., that p 1 p \ne{1} , our equation becomes

1 z = 3 2 z 2 z 2 3 z + 1 = 0 \dfrac{1}{z} = 3 - 2z \Longrightarrow 2z^{2} - 3z + 1 = 0 ,

which has solutions z = 1 z = 1 and z = 1 2 z = \frac{1}{2} . Now with z = 1 z = 1 we have that

log q ( p ) = 1 q = p x = 4 \log_{q}(p) = 1 \Longrightarrow q = p \Longrightarrow x = -4 ,

which would give us negative bases, and hence we can discard this value of z z . With z = 1 2 z = \frac{1}{2} , we have that

log q ( p ) = 1 2 p = q \log_{q}(p) = \frac{1}{2} \Longrightarrow p = \sqrt{q} .

After substituting for p p and q q , squaring both sides and simplifying, we end up with 4 x 2 + 9 x + 2 = 0 4x^{2} + 9x + 2 = 0 , which has solutions x = 2 x = -2 and x = 1 4 x = -\frac{1}{4} . The first of these makes p < 0 p \lt 0 , so we are left with just x = 1 4 x = -\frac{1}{4} , which does check out when plugging it back into the original equation.

Thus we have a = 1 , b = 4 a = 1, b = 4 , giving us the solution a + b = 5 a + b = \boxed{5} .

@Michael Mendrin Sorry, I didn't mean to duplicate your solution. When I started composing my solution there were no solutions posted yet, so I thought mine would be the only one. Otherwise, I wouldn't have gone to the trouble, (I'm still slow at this LaTeX gig :) ).

Brian Charlesworth - 6 years, 11 months ago

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Brian, the fun of mathematics are all the different ways of solving a problem. I can see the similarities between our approaches, but there's still enough differences to make reading interesting.

This was an really awkward problem for me.

Michael Mendrin - 6 years, 11 months ago

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Good point. Variety is always welcome. :) Thanks for replying.

Brian Charlesworth - 6 years, 11 months ago

Yes indeed. Thanks for your solutions :D

Priyansh Sangule - 6 years, 11 months ago

A perfect solution indeed ! :D Voted you up ! :)

Priyansh Sangule - 6 years, 11 months ago

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Thanks, Priyansh, and thanks also for posting the question. :)

Brian Charlesworth - 6 years, 11 months ago

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You're welcome buddy :D

Priyansh Sangule - 6 years, 11 months ago

Nicely put. Congratulations. I did the same way. But used change of base instead of your step on the third line. Your is better.

Niranjan Khanderia - 6 years, 11 months ago

Change of bases really did the trick faster!!!!Good problem.

Anubhab Ghosh - 6 years, 10 months ago

The question does not mention that we have to consider only real solutions, if we take x = 4 x=-4 that would result in complex function, but it would still satisfy it.

Aneesh Kundu - 6 years, 7 months ago
Michael Mendrin
Jul 13, 2014

We rearrange the equation, factorizing and using the notation

L o g a ( x ) = L o g [ a , x ] { Log }_{ a }(x)=Log[a,x] , and then do the following steps

L o g [ 2 x + 3 , ( 3 x + 7 ) ( 2 x + 3 ) ] 3 = 1 L o g [ 3 x + 7 , ( 2 x + 3 ) 2 ] Log[2x+3,(3x+7)(2x+3)]-3=1-Log[3x+7,{ (2x+3) }^{ 2 }]

L o g [ 2 x + 3 , ( 3 x + 7 ) ( 2 x + 3 ) ] L o g [ 2 x + 3 , ( 2 x + 3 ) 3 ] = Log[2x+3,(3x+7)(2x+3)]-Log[2x+3,{ (2x+3) }^{ 3 }]=

L o g [ 3 x + 7 , 3 x + 7 ] L o g [ 3 x + 7 , ( 2 x + 3 ) 2 ] Log[3x+7,3x+7]-Log[3x+7,{ (2x+3) }^{ 2 }]

L o g [ 2 x + 3 , 3 x + 7 ( 2 x + 3 ) 2 ] = L o g [ 3 x + 7 , 3 x + 7 ( 2 x + 3 ) 2 ] Log[2x+3,\dfrac { 3x+7 }{ { (2x+3) }^{ 2 } } ]=Log[3x+7,\dfrac { 3x+7 }{ { (2x+3) }^{ 2 } } ]

which means that

3 x + 7 ( 2 x + 3 ) 2 = 1 \dfrac { 3x+7 }{ { (2x+3) }^{ 2 } } =1

which leads to a quadratic equation with two roots x = 2 , 1 4 x=-2,-\dfrac { 1 }{ 4 } , but x = 2 x=-2 results in a negative base and delivers indeterminate results, leaving x = 1 4 = a b x=-\dfrac { 1 }{ 4 } =-\dfrac { a }{ b } as the only possible root, and thus a + b = 5 a+b=5

The magic is in the second step ;)

Aakarshit Uppal - 6 years, 11 months ago

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Yes, a propitous trick, wasn't it?

Michael Mendrin - 6 years, 11 months ago

Nice solution MIchael ! :D Voted you up ! :D

Priyansh Sangule - 6 years, 11 months ago
Gian Mark Pulgar
Jul 24, 2014

Get the factors of each trinomial, you will get log(3x+7)(2x+3) base (2x+3) = 4 - 2log(2x+3). Then apply properties of logarithms, you'll arrive at:

log (3x+7) base (2x+3) +1 = 4-2log(2x+3) base (3x+7)

Apply another property of logarithm, which is the change of base, the equation becomes:

log(3x+7)/log (2x+3) + 2log(2x+3)/log(3x+7) = 3

Recall: 2log(2x+3)/log(3x+7) = 1/ [2log(2x+3)/log(3x+7)]

this becomes log(3x+7)/2log (2x+3)

log(3x+7)/log (2x+3) + log(3x+7)/2log (2x+3) = 3

Combine the two fractions; this becomes

3log(3x+7)/2log (2x+3) = 3

3 is cancelled and 2log (2x+3) is transposed to the other side of the equation.

log(3x+7) = 2log (2x+3)

log is cancelled and the two becomes the exponent of (2x+3)

3x+7 = 4x^2+12x+9

Combine like terms and rearrange:

4x^2+9x+2=0 <---- get the factors of this trinomial, this decomposes into:

(4x+2)(x+1)=0

Values of x are -1/4 and -1, but -1 is an extraneous root. So -1/4 is the answer and the sum of its digits is 1+4 = 5

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