Biquadratic

Algebra Level 5

Solve for real x x

( 12 x 1 ) ( 6 x 1 ) ( 4 x 1 ) ( 3 x 1 ) = 5 (12x-1)(6x-1)(4x-1)(3x-1) = 5

If α \alpha and β \beta are its zeroes, then enter your answer as 1 α β \dfrac1{\alpha\beta} .


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The answer is -24.

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1 solution

Ankit Kumar Jain
Apr 21, 2017

We can rewrite the equation as 2 ( 12 x 1 ) 4 ( 6 x 1 ) 6 ( 4 x 1 ) 8 ( 3 x 1 ) = 5 2 4 6 8 2(12x-1)4(6x-1)6(4x-1)8(3x-1) = 5\cdot2\cdot4\cdot6\cdot8

( 24 x 2 ) ( 24 x 4 ) ( 24 x 6 ) ( 24 x 8 ) = 1920 \Rightarrow (24x-2)(24x-4)(24x-6)(24x-8) = 1920

Let \quad 24 x 5 = a 24x - 5 = a

Then , our equation becomes ( a 3 ) ( a 1 ) ( a + 1 ) ( a + 3 ) = 1920 (a-3)(a-1)(a+1)(a+3) = 1920

( a 2 1 ) ( a 2 9 ) = 1920 \Rightarrow (a^2-1)(a^2-9) = 1920

a 4 10 a 2 1911 = ( a 2 + 39 ) ( a 2 49 ) = 0 \Rightarrow a^4-10a^2-1911=(a^2+39)(a^2-49)=0

a 2 = 39 , 49 \Rightarrow a^2=-39,49

But since we want real solutions , we reject a 2 = 39 a^2=-39

a 2 = 49 a = ± 7 24 x 5 = ± 7 \Rightarrow a^2=49 \Rightarrow a = \pm 7 \Rightarrow 24x-5=\pm7

x = 1 2 , 1 12 \Rightarrow \boxed{x=\dfrac12 , \dfrac{-1}{12}}


Required answer is 24 \boxed{-24}

Nice solution (+1)

Sorry for that.

Rahil Sehgal - 4 years, 1 month ago

Don't say a sorry for just that.! By the way , how did you solve the problem? Same method?

Ankit Kumar Jain - 4 years, 1 month ago

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I did it by the long method. Multiplying all and then factorise and all...

Your method is short and nice. :)

Rahil Sehgal - 4 years, 1 month ago

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But , that factorization would have been pretty tough. Just like the problem , even the solution is also not mine. :( :(

Ankit Kumar Jain - 4 years, 1 month ago

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@Ankit Kumar Jain Yes.. that actually makes the problem hard. ( Level 4)

Rahil Sehgal - 4 years, 1 month ago

How is anyone supposed to even think like that? Did you just randomly multiplied numbers or there was any insight

Shantanu Saxena - 4 years, 1 month ago

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The motive was to convert that biquadratic to some quadratic. This can be done in this way that you are somehow able to use this identity ( a + b ) ( a b ) = a 2 b 2 (a+b)(a-b) = a^2-b^2 .

So , we need to make the coeff's of x in all the terms equal and then let some term to be a a so that we can get a quadratic in a 2 a^2 .

I hope that it clarifies your doubt..:):)

Ankit Kumar Jain - 4 years, 1 month ago

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And the question seemed to be quite defined in this way because after you make the coefficients equal you can easily see the term which you require to as a a

Ankit Kumar Jain - 4 years, 1 month ago

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