# The right digit of 4th power of any natural number

1,887

## Solution 1

Using congruent modulo 10 $$x \equiv 0,1,2,3,...,8,9 \,\ (mod \,\ 10)$$ or, $$x \equiv 0,\pm1,\pm2,\pm3,\pm4,\pm5 \,\ (mod \,\ 10)$$ or, $$x^2 \equiv 0,1,4,9,16,25 \,\ (mod \,\ 10)$$ or, $$x^2 \equiv 0,1,4,9,6,5 \,\ (mod \,\ 10)$$ or, $$x^2 \equiv 0,\pm1,4,5,6 \,\ (mod \,\ 10)$$ or, $$x^4 \equiv 0,1,16,25,36 \,\ (mod \,\ 10)$$ or, $$x^4 \equiv 0,1,6,5,6 \,\ (mod \,\ 10)$$ or, $$x^4 \equiv 0,1,6,5 \,\ (mod \,\ 10)$$ $=>$ the rightmost digit of $x^4$ is either $0,1,6$ or $5$.

## Solution 2

For the last digit of a product, you have to know only the last digits of the factors. Considering fourth powers, you only have 10 cases:

• 1: $1^4 = 1$
• 2: $2^4 = 16$
• 3: $3^4 = 81$
• 4: $4^4 = 256$
• 5: $5^4 = 625$
• 6: $6^4 = 1296$
• 7: $7^4 = 2401$
• 8: $8^4 = 4096$
• 9: $9^4 = 6561$
• 0: $0^4 = 0$

Looking at the last digits of these numbers, you see that only 0, 1, 5 and 6 appear. The last digits of these numbers have to be the last digit of any fourth power, for the reason stated above.

Share:
1,887