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Math Help - Find (10^5)^101 (mod 21)

  1. #1
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    Find (10^5)^101 (mod 21)

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  2. #2
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    Re: Find (10^5)^101 (mod 21)

    Hello, jackGee!

    \text{Find: }\: (10^5)^{101} \text{ (mod 21)}

    We find that:

    . . \begin{array}{cccc}10^0 &\equiv & 1 & \text{(mod 21)} \\ 10^1 &\equiv& 10 & \text{(mod 21)} \\ 10^2 &\equiv& 16 & \text{(mod 21)} \\ 10^3 &\equiv & 13 & \text{(mod 21)} \\ 10^4 &\equiv& 4 & \text{(mod 21)} \\ 10^5 &\equiv& 19 & \text{(mod 21)} \\ 10^6 &\equiv& 1 & \text{(mod 21)} \end{array}


    Therefore: . 10^{505} \;=\;10^{6\cdot84+1}

    n . . . . . . . . . . . . . =\;(10^6)^{84}\cdot 10^1

    n . . . . . . . . . . . . . \equiv\:1^{84}\cdot 10\text{ (mod 21)}

    . . . . . . . . . . . . . . \equiv\:10\text{ (mod 21)}
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    Re: Find (10^5)^101 (mod 21)

    Is there another way to do this, like using Eulidean algorithm ?
    because i cant use a calculator on my tests so for an example theres no way i can find 10^5 mod 21
    thanks!!!
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  4. #4
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    Re: Find (10^5)^101 (mod 21)

    You can use Soroban's method without needing a calculator. The trick is that if you know the mod(21) value for a certain power of ten, you can easily find mod(21) for the next higher power of ten - just multiply 10 by the remainder and find teh mod(21) value of that. Here you know 1 = 1 (mod 21). Multiply by the remainder of 1 by 10 and you find 10^1 = 10(mod 21). Now multiply th eremainder of 10 by 10 again and you get 10^2 = 16 mod 21. Continue for 10^3: 10 x 16 = 13 (mod 21). For 10^4: 10 x 13 = 4 mod(21). For 10^5: 10x4 = 19 mod(21). Then for 10^6: 190 = 1 mod(21). And then the pattern repeats.
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    Re: Find (10^5)^101 (mod 21)

    Here's yet another way to cut down on the arithmetic. 10^{505}=3^{505}=3^{6\cdot 84+1}=3 mod 7 and 10^{505}=1^{505}=1 mod 3. So by the Chinese remainder theorem, 10^{505}=10 mod 21.
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