Can you guess what I guessed!

Algebra Level 5

x 4 22 x 2 12 x + 22 = 0 x^{4}-22x^{2}-12x+22=0 The above equation has roots of the form ± a ± b ± c \pm \sqrt{a} \pm \sqrt{b \pm \sqrt{c}} , where a , b a, b and c c are constant positive integers , and a a and b 3 \sqrt[3]{b} are relatively prime.

Find a + b + c a+b+c .


The answer is 14.

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1 solution

Manuel Kahayon
May 26, 2016

The coefficients of the polynomial above hint at the equation being expressible in the form ( x 2 k ) 2 = a ( x r ) 2 (x^2-k)^2 = a(x-r)^2 for constants a , k , r a,k,r . We just take the square root of both sides and Voila! We are done.

But, how do we do it? We can use trial and error to get ( x 2 5 ) 2 = 3 ( 2 x + 1 ) 2 (x^2-5)^2 = 3(2x+1)^2 , but how can we do an algebraic approach?

We can express the equation above in the form x 4 a x 2 + ( 22 + b ) = ( 22 a ) x 2 + 12 x + b x^4 -ax^2+(22+b) = (22-a)x^2+12x+b , where a a and b b are constants to be determined.

Now, in order for the LHS to be a perfect square trinomial in x 2 x^2 , we need a 2 4 = 22 + b \frac{a^2}{4} = 22+b , or 4 b = a 2 88 4b = a^2 -88 . Similarly, for the RHS to be a multiple of a perfect square trinomial, its discriminant must be equal to 0 0 , or 144 ( 4 b ) ( 22 a ) = 0 144-(4b)(22-a) = 0 , but by the equation above, 4 b = a 2 88 4b = a^2 -88 , 144 ( a 2 88 ) ( 22 a ) = 0 144-(a^2 -88)(22-a) = 0 .Solving for a a gives us a = 10 a=10 , which consequently gives us b = 3 b = 3 .

Then, we can get x 4 10 x 2 + 25 = 3 ( 4 x 2 + 4 x + 1 ) x^4 -10x^2+25 = 3(4x^2+4x+1) , or similarly, ( x 2 5 ) 2 = 3 ( 2 x + 1 ) 2 (x^2-5)^2 = 3(2x+1)^2 , taking square root of both sides gives us

x 2 5 = ± 2 x 3 ± 3 x^2-5 = \pm 2x\sqrt{3} \pm \sqrt{3} , or equivalently, x 2 ± 2 x 3 5 ± 3 = 0 x^2 \pm 2x\sqrt{3} -5 \pm \sqrt{3}=0 . We then use the quadratic formula to obtain

x = ± 3 ± 8 ± 3 x = \pm \sqrt{3} \pm \sqrt{8 \pm \sqrt{3}} .

Therefore, a = 3 a = 3 , b = 8 b = 8 and c = 3 c = 3 , giving us a + b + c = 14 a+b+c = \boxed{14}

Nicely done (+1)

Department 8 - 5 years ago

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Thank you :)

Manuel Kahayon - 5 years ago

Good job @Manuel Kahayon .

I was looking for this kind of solution. One can always resort to standard algebraic methods to solve problems, but there is always an other way around and you found it.

I'm interested to know how did you infer from the co-efficients that the equation is expressible in the form... .

Aditya Sky - 5 years ago

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Thank you :)

I actually guessed that the equation will be expressible in the form by instinct, since if you transpose the 12 x -12x you will get an equation in x 2 x^2 in the LHS...

Manuel Kahayon - 5 years ago

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Got it :)..

Aditya Sky - 5 years ago

But you have stated in your question that a 1 / 3 a^{1/3} and b b are relatively prime. But such values of a and b do not satisfy so.

I got the same but due to this statement i opted for " discuss solution ".

Change it as : b 1 / 3 b^{1/3} and a a are relatively prime.

Aakash Khandelwal - 5 years ago

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I'm extremely sorry for that.

Aditya Sky - 5 years ago

Nicely done

Gaurav Chahar - 5 years ago

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