Question

(Euler's Theorem, 5pt) What is the last digit of 7^8984392344350386 (in its decimal expansion)? Explain how you did it. Hint: can you reexpress "last digit" more mathematically, so you can apply Euler's theorem? Hint 2: you can do this whole problem in your head. No calculator required, just thinking.

Answer 1

To apply Euler's Theorem, let's first reexpress "last digit" more mathematically as "the remainder when the number is divided by 10". Then, we can use the fact that Euler's Theorem states that if a and n are coprime positive integers, then a^φ(n) ≡ 1 (mod n), where φ is Euler's totient function. Since 7 and 10 are coprime, we have φ(10) = 4, so 7^φ(10) ≡ 1 (mod 10), which means that 7^4 ≡ 1 (mod 10).

Now, we can use this fact to reduce the exponent 8984392344350386 modulo 4, since any power of 7 that is a multiple of 4 will have the same remainder when divided by 10 as 7^0 = 1. Since 8984392344350386 is clearly even, we have 7^8984392344350386 ≡ 7^0 ≡ 1 (mod 10). Therefore, the last digit of 7^8984392344350386 is 1.

In summary: The last digit of 7^8984392344350386 is 1, which was obtained by reexpressing "last digit" as "remainder when divided by 10", applying Euler's Theorem to reduce the exponent modulo 4, and using the fact that any power of 7 that is a multiple of 4 will have the same remainder when divided by 10 as 7^0, which is 1.

Step-by-step explanation: