Exercise · ZW-MODULAR-INVERSE-007 · ★★☆ Intermediate
Which pair of numbers are inverses of each other modulo \(10\)?
Choose one option.
Show answer
- B. \(3\) and \(7\)
Explanation
Since \(3\cdot 7=21=2\cdot 10+1\), the numbers \(3\) and \(7\) are inverses of each other. The other pairs fail: \(2\cdot 5\equiv 0\) and \(4\cdot 4\equiv 6\pmod{10}\).
Review the concept: Modular inverse
The modular inverse of an integer \(a\) modulo \(m\) is an integer \(b\) whose product with \(a\) is congruent to 1 modulo \(m\).
\[ab \equiv 1 \pmod{m}\]
More practice problems: modular inverse
- ZW-MODULAR-INVERSE-001 What is the necessary and sufficient condition for a to have an inverse modulo m?
- ZW-MODULAR-INVERSE-002 Does 2 have an inverse modulo 4?
- ZW-MODULAR-INVERSE-003 State the uniqueness property of the modular inverse modulo m.
- ZW-MODULAR-INVERSE-004 Which integer b satisfies 3b≡ 1pmod5?
- ZW-MODULAR-INVERSE-005 Which of these congruences has a solution in b?
- ZW-MODULAR-INVERSE-006 Find the inverse of 5 modulo 11.
- ZW-MODULAR-INVERSE-008 Solve 3x≡ 2pmod7 using the inverse of 3.
- ZW-MODULAR-INVERSE-009 If b is an inverse of a modulo m and c≡ apmodm, what holds?