Problem C. 1462. (February 2018)
C. 1462. The first term of an arithmetic sequence is \(\displaystyle a_1=3\), and its common difference is 9. Prove that for every natural number \(\displaystyle k\), the number \(\displaystyle 3\cdot 4^k\) occurs among the terms.
(5 pont)
Deadline expired on March 12, 2018.
Sorry, the solution is available only in Hungarian. Google translation
Megoldás. A sorozat \(\displaystyle n\)-edik eleme: \(\displaystyle a_n=3+(n-1)\cdot9=3+9m=3(1+3m)\). Bizonyítandó, hogy tetszőleges természetes \(\displaystyle k\) számra van olyan \(\displaystyle m\) pozitív egész, melyre \(\displaystyle 3(1+3m)=3\cdot4^k\). Az egyenlet mindkét oldalát \(\displaystyle 3\)-mal osztva és levonva \(\displaystyle 1\)-et: \(\displaystyle 3m=4^k-1\). A jobb oldali kifejezés minden \(\displaystyle k\) egész szám esetén szorzattá alakítható: \(\displaystyle 3m=4^k-1^k=(4-1)(4^{k-1}+4^{k-2}+...+4+1)\). Vagyis ha \(\displaystyle m=4^{k-1}+4^{k-2}+...+4+1\), akkor \(\displaystyle a_{9m+3}=3\cdot4^k\).
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100 students sent a solution. 5 points: 57 students. 4 points: 18 students. 3 points: 11 students. 2 points: 3 students. 1 point: 8 students. 0 point: 3 students.
Problems in Mathematics of KöMaL, February 2018