I and Phi?

Algebra Level 5

210 + 315 ϕ + 76 i ϕ 3 x + 34 ϕ 2 x 2 + 4 i ϕ x 3 + x 4 = 0 \large 210+315\phi+76i\phi^3x+34\phi^2x^2+4i\phi x^3+x^4=0

The equation above has 4 distinct roots a n a_n for n = 1 , 2 , 3 , 4 n={1,2,3,4} where a 1 < a 2 < a 3 < a 4 |a_1|<|a_2|<|a_3|<|a_4| .

Find the value of a 1 + a 2 a 3 a 4 \dfrac{a_1+a_2-a_3}{a_4} .

Details and assumptions :

i = 1 i=\sqrt{-1} , ϕ = 1 + 5 2 \phi=\dfrac{1+\sqrt5}{2} .


The answer is 1.

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1 solution

Trevor Arashiro
May 17, 2015

I made this problem to prove that this factoring trick can be used for polys of all degrees.

Let's look at the constant term. Using the identity, ϕ n = ϕ n 1 + ϕ n 2 \phi^n=\phi^{n-1}+\phi^{n-2} .

210 + 315 ϕ = 105 + 105 ϕ + 105 ϕ + 105 + 105 ϕ 210+315\phi=105+105\phi+105\phi+105+105\phi

105 ϕ 2 + 105 ϕ 2 + 105 ϕ = 105 ϕ 2 + 105 ϕ 3 = 105 ϕ 4 \Longrightarrow 105\phi^2+105\phi^2+105\phi=105\phi^2+105\phi^3=105\phi^4

Now we have

105 ϕ 4 + 76 i ϕ 3 x + 34 ϕ 2 x 2 + 4 i ϕ x 3 + x 4 = 0 105\phi^4+76i\phi^3x+34\phi^2x^2+4i\phi x^3+x^4=0

Using this . We extract ϕ \phi and i i inversely with x x (aka, as the power of x x decreases, we factor out larger and larger powers of i i and ϕ \phi ) and are left with

105 76 x 34 x 2 + 4 x 3 + x 4 = 0 105-76x-34x^2+4x^3+x^4=0

Remainder theorem tells us that x = 1 x=1 is a root.

( x 1 ) ( x 3 + 5 x 2 29 x 105 ) = 0 (x-1)(x^3+5x^2-29x-105)=0

Once again, remainder theorem/rational roots tells us that x = 5 x=5 is a root.

( x 1 ) ( x 5 ) ( x 2 + 10 x + 21 ) = 0 (x-1)(x-5)(x^2+10x+21)=0

( x 1 ) ( x 5 ) ( x + 3 ) ( x + 7 ) = 0 (x-1)(x-5)(x+3)(x+7)=0

Factoring back in the i i and ϕ \phi

( x i ϕ ) ( x 5 i ϕ ) ( x + 3 i ϕ ) ( x + 7 i ϕ ) = 0 (x-i\phi)(x-5i\phi)(x+3i\phi)(x+7i\phi)=0

Thus a 1 = i ϕ , a 2 = 3 i ϕ , a 3 = 5 i ϕ , a 4 = 7 i ϕ a_1=i\phi,~a_2=-3i\phi,~a_3=5i\phi,~a_4=-7i\phi and

i ϕ 3 i ϕ 5 i ϕ 7 i ϕ = 1 \dfrac{i\phi-3i\phi-5i\phi}{-7i\phi}=\boxed{1}

+1 !

Nice trick ! (y)

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Thanks :).

Haha (y). Ikwym

Trevor Arashiro - 6 years ago

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