2 1 0 + 3 1 5 ϕ + 7 6 i ϕ 3 x + 3 4 ϕ 2 x 2 + 4 i ϕ x 3 + x 4 = 0
The equation above has 4 distinct roots a n for n = 1 , 2 , 3 , 4 where ∣ a 1 ∣ < ∣ a 2 ∣ < ∣ a 3 ∣ < ∣ a 4 ∣ .
Find the value of a 4 a 1 + a 2 − a 3 .
Details and assumptions :
i = − 1 , ϕ = 2 1 + 5 .
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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 .
2 1 0 + 3 1 5 ϕ = 1 0 5 + 1 0 5 ϕ + 1 0 5 ϕ + 1 0 5 + 1 0 5 ϕ
⟹ 1 0 5 ϕ 2 + 1 0 5 ϕ 2 + 1 0 5 ϕ = 1 0 5 ϕ 2 + 1 0 5 ϕ 3 = 1 0 5 ϕ 4
Now we have
1 0 5 ϕ 4 + 7 6 i ϕ 3 x + 3 4 ϕ 2 x 2 + 4 i ϕ x 3 + x 4 = 0
Using this . We extract ϕ and i inversely with x (aka, as the power of x decreases, we factor out larger and larger powers of i and ϕ ) and are left with
1 0 5 − 7 6 x − 3 4 x 2 + 4 x 3 + x 4 = 0
Remainder theorem tells us that x = 1 is a root.
( x − 1 ) ( x 3 + 5 x 2 − 2 9 x − 1 0 5 ) = 0
Once again, remainder theorem/rational roots tells us that x = 5 is a root.
( x − 1 ) ( x − 5 ) ( x 2 + 1 0 x + 2 1 ) = 0
( x − 1 ) ( x − 5 ) ( x + 3 ) ( x + 7 ) = 0
Factoring back in the i and ϕ
( x − i ϕ ) ( x − 5 i ϕ ) ( x + 3 i ϕ ) ( x + 7 i ϕ ) = 0
Thus a 1 = i ϕ , a 2 = − 3 i ϕ , a 3 = 5 i ϕ , a 4 = − 7 i ϕ and
− 7 i ϕ i ϕ − 3 i ϕ − 5 i ϕ = 1