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Algebra Difficulty 6.0 AIME, harder Prove it

9. nn is a non-negative integer, express (1+423443)(1+4 \sqrt[3]{2} -4 \sqrt[3]{4}) as (1+423443)(1+4 \sqrt[3]{2}-4 \sqrt[3]{4}) " =aa+ba23+ca43=a_{a}+b_{\mathrm{a}} \sqrt[3]{2}+c_{\mathrm{a}} \sqrt[3]{4}, where an,bn,caa_{\mathrm{n}}, b_{\mathrm{n}}, c_{\mathrm{a}} are integers. Prove: if cn=0c_{n}=0, then n=0n=0.

Solution

 9. Given (1+423443)n+1=(a0+b023+c043)(1+423443) \begin{array}{l} \text { 9. Given } (1+4 \sqrt[3]{2}-4 \sqrt[3]{4})^{n+1} \\ =\left(a_{0}+b_{0} \sqrt[3]{2}+c_{0} \sqrt[3]{4}\right)(1+4 \sqrt[3]{2} \\ -4 \sqrt[3]{4}) \end{array}

we get
an+1=a08bn+8cn a_{n+1}=a_{0}-8 b_{n}+8 c_{n}

Since a0=1a_{0}=1, all a0a_{0} are odd.
Every non-zero integer kk can be expressed as k=2pkk=2^{p} \cdot k', where kk' is odd and p0p \geqslant 0. Define v(k)=pv(k)=p. Let β0=v(bn),γ0=v(c0)\beta_{0}=v\left(b_{n}\right), \quad \gamma_{0}=v\left(c_{0}\right).
(1) For non-negative integer t,β2t=γ2t=t+2t, \beta_{2^{t}}=\gamma_{2 t}=t+2.
Prove: When t=0t=0, b1=4,c1=4b_{1}=4, c_{1}=-4.

The above assertion holds. Assume β2t=γ2t=t+2\beta_{2 t}=\gamma_{2 t}=t+2, then since
(a+2t+2(b23+c43))2=a2+2t+3a(b23+c43)+22t+4(b23+c43)2=A+2t+3(B23+C43). \begin{aligned} & \left(a+2^{t+2}(b \sqrt[3]{2}+c \sqrt[3]{4})\right)^{2} \\ = & a^{2}+2^{t+3} a(b \sqrt[3]{2}+c \sqrt[3]{4})+2^{2t+4} \\ & (b \sqrt[3]{2}+c \sqrt[3]{4})^{2} \\ = & A+2^{t+3}(B \sqrt[3]{2}+C \sqrt[3]{4}) . \end{aligned}

When a,b,ca, b, c are odd, A,B,CA, B, C are odd, thus β2t+1=γ2t+1=t+3\beta_{2^{t+1}}=\gamma_{2^{t+1}}=t+3, hence the assertion holds for all non-negative integers.
(2) If n,mn, m are integers, bn,cn,bm,cmb_{n}, c_{n}, b_{m}, c_{m} are non-zero, βn=γ0=λ,βm=γm=μ,μ<λ\beta_{n}=\gamma_{0}=\lambda, \beta_{m}=\gamma_{m}=\mu, \mu<\lambda, then bn+m,cn+mb_{n+m}, c_{n+m} are non-zero, and βn+m=γn+m=μ\beta_{n+m}=\gamma_{n+m}=\mu.
Prove: By (a+2λ(b23+c43))\left(a'+2^{\lambda}\left(b' \sqrt[3]{2}+c' \sqrt[3]{4}\right)\right)
(a+2μ(b23+c43))=A+2μ(B23+C43), \begin{aligned} \left(a''\right. & \left.+2^{\mu}\left(b'' \sqrt[3]{2}+c'' \sqrt[3]{4}\right)\right) \\ & =A+2^{\mu}(B \sqrt[3]{2}+C \sqrt[3]{4}), \end{aligned}

When a,a;b,b,c,ca', a''; b', b'', c', c'' are odd, AA, B,CB, C are all odd.
(3) For every integer n1n \geqslant 1, let
n=2tr+2tr1++2t1,0t1<<tr, \begin{array}{l} n=2^{t_{r}}+2^{t_{r-1}}+\cdots+2^{t_{1}}, \\ 0 \leqslant t_{1}<\cdots<t_{r}, \end{array}

then cnc_{n} is non-zero, and γn=t1+2\gamma_{n}=t_{1}+2.
Proof: This follows from (1) and (2).
From (3), the conclusion of the problem is established.
Note: b1080b_{1080} and c1980c_{1980} are divisible by 4 but not by 8.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.