Consider an arithmetic series and a geometric series having four initial terms from the set . If the last terms of these series are the maximum possible four digit numbers, then the number of common terms in these two series is equal to ____
Answer: 3
Solution
From the given set, are four terms of an arithmetic progression with common difference , and are four terms of a geometric progression with ratio .
Every term of the arithmetic progression leaves remainder on division by , and the largest four digit number of that form is , so the progression is . The geometric progression is , since while the next term has five digits.
The geometric terms are
and those leaving remainder on division by are , and ; each exceeds and is at most , so each belongs to the arithmetic progression as well.
The two series have terms in common.
Every term of the arithmetic progression leaves remainder on division by , and the largest four digit number of that form is , so the progression is . The geometric progression is , since while the next term has five digits.
The geometric terms are
and those leaving remainder on division by are , and ; each exceeds and is at most , so each belongs to the arithmetic progression as well.
The two series have terms in common.
JEE Main · 2021 · 16 Mar 2021 · Shift 1 · Integer answer