for Tawa, an old problem explained
(1) x+y+z=α
(2) x^2 +y^2 +z^2 =β
(3) x^3 +y^3 +z^3 =γ
α, β, γ are given
(4) x^4 +y^4 +z^4 =p
(5) x^5 +y^5 +z^5 =q
find p, q
we could try to solve the system but it′s hard
to exactly solve it and then calculate the
sums of the 4^(th) and 5^(th) powers.
instead, let′s think of the shape of the
solutions of a polynome of 3^(rd) degree. they
might look like this:
t_1 =a t_2 =b−(√c) t_3 =b+(√c)
let′s try it!
x=a
y=b−(√c)
z=b+(√c)
⇒
(1) a+2b=α
(2) a^2 +2b^2 +2c=β
(3) a^3 +2b^3 +6bc=γ
(4) a^4 +2b^4 +12b^2 c+2c^2 −p=0
(5) a^5 +2b^5 +20b^3 c+10bc^2 −q=0
doesn′t look that bad. we can easily solve
equation (1) for a and equation (2) for c
(1) a=α−2b
(2) c=−(a^2 /2)−b^2 +(β/2)=^([using (1)]) −3b^2 +2αb−(α^2 /2)+(β/2)
now let′s insert these in (3), (4) and (5)
(3) −24b^3 +24αb^2 −3(3α^2 −β)b+α^3 −γ=0
(4) −32αb^3 +32α^2 b^2 −4α(3α^2 −β)b+((3α^4 )/2)−α^2 β+(β^2 /2)−p=0
(5) −20(α^2 +β)b^3 +20α(α^2 +β)b^2 −(5/2)(3α^4 +2α^2 β−β^2 )b+α^5 −q=0
dividing by the constant factors of b^2
(3) b^3 −αb^2 +((3α^2 −β)/8)b−((α^3 −γ)/(24))=0
(4) b^3 −αb^2 +((3α^2 −β)/8)b+((2p−3α^4 +2α^2 β−β^2 )/(64α))=0
(5) b^3 −αb^2 +((3α^2 −β)/8)b+((q−α^5 )/(20(α^2 +β)))=0
only the red factors differ ⇒ they must have
the same values!
⇒ −((α^3 −γ)/(24))=((2p−3α^4 +2α^2 β−β^2 )/(64α))=((q−α^5 )/(20(α^2 +β)))
⇒
p=(α^4 /6)−α^2 β+((4αγ)/3)+(β^2 /2)
q=(α^5 /6)−((5α^3 β)/6)+((5α^2 γ)/6)+((5βγ)/6)
solved without solving
the same idea is trivial for 2 equations
x+y=α
x^2 +y^2 =β
x^n +y^n =p with n>2∧n∈N
put x=a−(√b) y=a+(√b)
⇒
2a=α
2a^2 +2b=β
⇒ a=(α/2) b=(β/2)−(α^2 /4)
we might as well solve the system
and for 4 equations unfortunately we must
solve the system
w+x+y+z=α
w^2 +x^2 +y^2 +z^2 =β
w^3 +x^3 +y^3 +z^3 =γ
w^4 +x^4 +y^4 +z^4 =δ
but it still makes it easier to solve putting
w=a−(√b) x=a+(√b) y=c−(√b) z=c+(√d)
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