Consider the following equation in ' x ' : lo g 2 x + 3 ( 6 x 2 + 2 3 x + 2 1 ) = 4 − lo g 3 x + 7 ( 4 x 2 + 1 2 x + 9 )
The sum of possible values of ' x ' satisfying the above equation is of the form b − a ; where a and b are co-prime integers.
Find a + b
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@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 :) ).
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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.
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Good point. Variety is always welcome. :) Thanks for replying.
Yes indeed. Thanks for your solutions :D
A perfect solution indeed ! :D Voted you up ! :)
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Thanks, Priyansh, and thanks also for posting the question. :)
Nicely put. Congratulations. I did the same way. But used change of base instead of your step on the third line. Your is better.
Change of bases really did the trick faster!!!!Good problem.
The question does not mention that we have to consider only real solutions, if we take x = − 4 that would result in complex function, but it would still satisfy it.
We rearrange the equation, factorizing and using the notation
L o g a ( x ) = L o g [ 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 ]
L o g [ 2 x + 3 , ( 3 x + 7 ) ( 2 x + 3 ) ] − L o g [ 2 x + 3 , ( 2 x + 3 ) 3 ] =
L o g [ 3 x + 7 , 3 x + 7 ] − L o g [ 3 x + 7 , ( 2 x + 3 ) 2 ]
L o g [ 2 x + 3 , ( 2 x + 3 ) 2 3 x + 7 ] = L o g [ 3 x + 7 , ( 2 x + 3 ) 2 3 x + 7 ]
which means that
( 2 x + 3 ) 2 3 x + 7 = 1
which leads to a quadratic equation with two roots x = − 2 , − 4 1 , but x = − 2 results in a negative base and delivers indeterminate results, leaving x = − 4 1 = − b a as the only possible root, and thus a + b = 5
The magic is in the second step ;)
Nice solution MIchael ! :D Voted you up ! :D
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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Let p = 2 x + 3 and q = 3 x + 7 . Then the given equation can be written as
lo g p ( p ∗ q ) = 4 − lo g q ( p 2 ) ⟹ 1 + lo g p ( q ) = 4 − 2 ∗ lo g q ( p ) .
Now by the change-of-base rule we have that lo g p ( q ) = lo g q ( p ) 1 .
So letting z = lo g q ( p ) , and assuming that z = 0 , i.e., that p = 1 , our equation becomes
z 1 = 3 − 2 z ⟹ 2 z 2 − 3 z + 1 = 0 ,
which has solutions z = 1 and z = 2 1 . Now with z = 1 we have that
lo g q ( p ) = 1 ⟹ q = p ⟹ x = − 4 ,
which would give us negative bases, and hence we can discard this value of z . With z = 2 1 , we have that
lo g q ( p ) = 2 1 ⟹ p = q .
After substituting for p and q , squaring both sides and simplifying, we end up with 4 x 2 + 9 x + 2 = 0 , which has solutions x = − 2 and x = − 4 1 . The first of these makes p < 0 , so we are left with just x = − 4 1 , which does check out when plugging it back into the original equation.
Thus we have a = 1 , b = 4 , giving us the solution a + b = 5 .