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Algebra Difficulty 5.3 AIME, harder Prove it Ireland

Suppose u,vu, v are distinct roots of the cubic polynomial
p(z)=z35z2+6z1. p(z) = z^3 - 5z^2 + 6z - 1.
Prove that u,vu, v, and u/vu/v are positive irrational real numbers.

Solution

p(0)=1, p(1)=1, p(2)=p(3)=1, p(4)=7, p(0) = -1,\ p(1) = 1,\ p(2) = p(3) = -1,\ p(4) = 7,
the roots of pp are positive real numbers that belong to the union of the intervals (0,1)(0, 1), (1,2)(1, 2) and (3,4)(3, 4). Hence the roots are distinct, and neither of them is an integer. But since the coefficients of pp are integers, neither can they be rational numbers, by the Gauss Lemma. Hence, u,vu, v are irrational. Suppose now that u=vru = v r, where rr is rational, and r1r \ne 1. Then vv satisfies the pair of cubic equations:
v35v2+6v1=0, r3v35r2v2+6rv1=0. v^3 - 5v^2 + 6v - 1 = 0,\ \quad r^3 v^3 - 5r^2 v^2 + 6r v - 1 = 0.
Eliminating the term involving v3v^3 in the usual way, we see that
(1r)(5r2v26r(1+r)v+1+r+r2)=0. (1 - r)(5r^2 v^2 - 6r(1 + r)v + 1 + r + r^2) = 0.
Since r1r \ne 1 by hypothesis, it follows that uu satisfies the quadratic equation
5u2=6(r+1)u(1+r+r2), 5u^2 = 6(r + 1)u - (1 + r + r^2),
whence (multiplying by 5u5u and substituting for 5u25u^2)
25u3=6(r+1)5u25(1+r+r2)u=(36(r+1)25(1+r+r2))u6(r+1)(1+r+r2), \begin{aligned} 25u^3 &= 6(r + 1)5u^2 - 5(1 + r + r^2)u \\ &= (36(r + 1)^2 - 5(1 + r + r^2))u - 6(r + 1)(1 + r + r^2), \end{aligned}
i.e.,
25u3=(31+67r+31r2)u6(r+1)(1+r+r2). 25u^3 = (31 + 67r + 31r^2)u - 6(r + 1)(1 + r + r^2).
Substituting these expressions for u3,u2u^3, u^2 into u35u2+6u1=0u^3 - 5u^2 + 6u - 1 = 0, we derive a linear equation for uu with non-zero rational coefficients:
0=(3183r+31r2)u(1+r)(619r+6r2). 0 = (31 - 83r + 31r^2)u - (1 + r)(6 - 19r + 6r^2).
This implies that uu is rational, which is false. This contradiction establishes the conclusion that u/vu/v is also irrational.

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