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Q66·CSAT · Prelims 2025

Distinct remainders of 1^n + 2^n mod 4

NumericalRemainders & CyclicityNumber theoryMedium

Question

If n is a natural number, then what is the number of distinct remainders of (1^n + 2^n) when divided by 4?

Options

a

0

b

1

c

2

Answer
d

3

Explanation

Evaluate (1^n + 2^n) \bmod 4 for consecutive natural numbers n: For n = 1: (1^1 + 2^1) = 3. 3 \bmod 4 = 3. For n = 2: (1² + 2²) = 5. 5 \bmod 4 = 1. For n = 3: (1³ + 2³) = 9. 9 \bmod 4 = 1. For n = 4: (1^4 + 2^4) = 17. 17 \bmod 4 = 1. For all ⟨MATH⟩n \ge 2⟨/MATH⟩, ⟨MATH⟩2^n⟨/MATH⟩ is a multiple of 4. Therefore, 2^n \bmod 4 = 0. Since 1^n is always 1, the expression simplifies to 1 + 0 = 1 for all higher powers. The only generated remainders are 3 (when n=1) and 1 (when n \ge 2). This yields exactly 2 distinct remainders.

For modular arithmetic expressions containing k^n \bmod m, once n is large enough that k^n is a multiple of m, that term permanently drops to 0.

Answer: (c).

Question details

Year

2025

Paper

CSAT

Question

Q66

Section

Numerical Ability

Sub-topic

Remainders & Cyclicity

Type

Number theory

Difficulty

Medium

Source hint

Number theory

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