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RolyPoly

Why is the last two digits of \(91^{71}\) the same as 91mod100? Similarly, why is it true that the last two digits of \(81^{81}\) the same as 81mod100, and that of \(71^{91}\) the same as 71mod100?

  • 9 months ago
  • 9 months ago

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  1. RolyPoly
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    Maybe I should edit my question a bit. It should be "how do I know if the last two digits of \(91^{71}\) is the same as 91mod100? What if I need to find last two digits of\(91^{70}\)?

    • 9 months ago
  2. Jack17
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    there the same thing, how much modular arithmetic do you know

    • 9 months ago
  3. RolyPoly
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    I only know how to find x^y mod (n) using the trick that express y in binary number first, then take mod.

    • 9 months ago
  4. RolyPoly
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    Why are you deleting your comment?

    • 9 months ago
  5. Jack17
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    oh your refering to the binary exponentiation algorithm

    • 9 months ago
  6. RolyPoly
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    This one: http://openstudy.com/updates/51165c4de4b09e16c5c86007

    • 9 months ago
  7. Jack17
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    $$91^{71}\equiv 91 \text{ mod 100}$$ is what you are trying to show

    • 9 months ago
  8. RolyPoly
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    Wait!

    • 9 months ago
  9. Jack17
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    ok

    • 9 months ago
  10. RolyPoly
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    Suppose the question is find the last two digits of \(91^{71}\). How would you start?

    • 9 months ago
  11. Jack17
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    Do you know what the mod operator does?

    • 9 months ago
  12. Jack17
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    if I were to write 91^71 in base 10

    • 9 months ago
  13. Jack17
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    it would be $$91^{71}=a+b10+c10^2+d10^3...$$

    • 9 months ago
  14. Jack17
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    now we can reduce that modulo 100, ok

    • 9 months ago
  15. RolyPoly
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    Ehh!!! So, finding the last two digits of n is the same as finding n mod 100?

    • 9 months ago
  16. Jack17
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    so $$91^{71}\text{ mod 100}=a+10b$$ Where a is are first digit and b are second digit

    • 9 months ago
  17. Jack17
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    yes pretty much

    • 9 months ago
  18. RolyPoly
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    And finding the last n digits of m is the same as finding m mod (10^n)?

    • 9 months ago
  19. Jack17
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    yes

    • 9 months ago
  20. Jack17
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    Which you can compute using your exponentiation by squaring algorithm or using a variety of other techniques

    • 9 months ago
  21. Jack17
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    though for small numbers like yours, I wouldn't use that technique

    • 9 months ago
  22. RolyPoly
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    What technique would you use?

    • 9 months ago
  23. RolyPoly
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    Don't tell me calculator ~_~

    • 9 months ago
  24. Jack17
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    well for $$91^{71}\text{ mod 100}$$

    • 9 months ago
  25. Jack17
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    I would reduce the 91 modulo 100, so I get,

    • 9 months ago
  26. Jack17
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    $$91^{71} \text{ mod 100} = (-9)^{71} \text{ mod 100}= -9^{71} \text{ mod 100}$$

    • 9 months ago
  27. RolyPoly
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    "reduce the 91 modulo 100"?

    • 9 months ago
  28. Jack17
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    Yes, I think maybe you should read up a little on modular arithmetic,

    • 9 months ago
  29. Jack17
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    $$91\equiv -9 \text{ mod 100}$$

    • 9 months ago
  30. Jack17
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    therefore

    • 9 months ago
  31. Jack17
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    $$91^{71}\equiv (-9)^{71}\text{ mod 100}$$

    • 9 months ago
  32. Jack17
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    so now we just have to compute -9 to the 71st power modulo 100

    • 9 months ago
  33. Jack17
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    but $$(-1)^{71}=-1$$

    • 9 months ago
  34. Jack17
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    so its just computing $$-9^{71}\text{ mod 100}$$

    • 9 months ago
  35. Jack17
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    Now 9 is coprime to 100

    • 9 months ago
  36. Jack17
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    and $$\phi(9)=6$$

    • 9 months ago
  37. Jack17
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    $$9^{6}\equiv 1 \text{ mod 100}$$

    • 9 months ago
  38. Jack17
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    raising both sides to the 12th power, gives

    • 9 months ago
  39. Jack17
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    $$9^{72}\equiv 1 \text{ mod 100 }$$

    • 9 months ago
  40. Jack17
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    $$-9^{72}\equiv -1 \text{ mod 100}$$

    • 9 months ago
  41. Jack17
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    but wqe want $$-9^{71} \text{ mod 100}$$

    • 9 months ago
  42. Jack17
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    if we let $$-9^{71}=x$$

    • 9 months ago
  43. Jack17
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    we have $$9x\equiv -1 \text{ mod 100}$$

    • 9 months ago
  44. Jack17
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    now all we have to do is solve for x modulo 100, and we have are solution, I know this looked like it took a while but its because I had to type over this chat, also there is many ways to solve somthing like this, if your just using a computer an algorithm like yours might be better.

    • 9 months ago
  45. Jack17
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    But if your doing it by hand, its helpful to be familear with some of this stuff, because sometimes there are 'easyier' ways to do things.

    • 9 months ago
  46. RolyPoly
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    I'll tag you if I have questions. I need to work it out myself to see if I really understand it. Sleep well :)

    • 9 months ago
  47. Jack17
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    alright later

    • 9 months ago
  48. RolyPoly
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    @Jack17 Problem problem problem "Now 9 is coprime to 100" "and ϕ(9)=6" " \(9^6≡1 mod 100\)" But isn't it \(a^{\phi(n)} ≡ 1 (\mod n)\) ? Why would we find ϕ(9)???

    • 9 months ago
  49. Jack17
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    nice catch, this changes how you would proceed

    • 9 months ago
  50. Jack17
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    We want $$\phi(100)$$

    • 9 months ago
  51. Jack17
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    which is 40

    • 9 months ago
  52. RolyPoly
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    Don't be so lazy... :(

    • 9 months ago
  53. RolyPoly
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    Yay!!! Thanks!! :)

    • 9 months ago
  54. Jack17
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    $$9^{10}\equiv 1 \text{ mod 100 }$$

    • 9 months ago
  55. Jack17
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    Which you can get by exponentiation by squaring

    • 9 months ago
  56. Jack17
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    nvm im being stupid the original problem was $$-9^{71} \text{ mod 100}$$

    • 9 months ago
  57. Jack17
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    this is the same as $$-3^{142} \text{ mod 100}$$

    • 9 months ago
  58. Jack17
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    ahh maybe you should use your algorithm, i am to lazy to figure this out

    • 9 months ago
  59. RolyPoly
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    142 is a LARGE number!!!

    • 9 months ago
  60. Jack17
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    then use your technique

    • 9 months ago
  61. Jack17
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    use the fact $$9^{10}\equiv 1 \text{ mod 100}$$

    • 9 months ago
  62. Jack17
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    so $$9^{70}\equiv 1 \text{ mod 100}$$

    • 9 months ago
  63. Jack17
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    $$9^{71}\equiv 9\text{ mod 100}$$

    • 9 months ago
  64. RolyPoly
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    9^(40) ≡1 mod 100 since phi(100) is 40 o_o

    • 9 months ago
  65. Jack17
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    $$-9^{71}\equiv -9 \text{ mod 100}$$

    • 9 months ago
  66. Jack17
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    $$-9^{71}\equiv 91 \text{ mod 100}$$

    • 9 months ago
  67. Jack17
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    $$91^{71}\equiv 91 \text{ mod 100}$$

    • 9 months ago
  68. Jack17
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    the first two digits of $$91^{71}$$ are 1 and 9

    • 9 months ago
  69. RolyPoly
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    9^(10) ≡1 mod 100 <- How do you work this out?

    • 9 months ago
  70. Jack17
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    $$9^2\equiv -19 \text{ mod 100}$$

    • 9 months ago
  71. Jack17
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    $$9^4\equiv 19^2 \text{ mod 100}$$

    • 9 months ago
  72. Jack17
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    $$9^4\equiv -39 \text{ mod 100}$$

    • 9 months ago
  73. Jack17
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    $$9^8 \equiv 39^2 \text{ mod 100}$$

    • 9 months ago
  74. Jack17
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    $$9^8\equiv 21 \text{ mod 100}$$

    • 9 months ago
  75. Jack17
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    $$9^{10}\equiv 21*9^2 \text{ mod 100}$$

    • 9 months ago
  76. Jack17
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    $$9^{10}\equiv 1 \text{ mod 100}$$

    • 9 months ago
  77. RolyPoly
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    Got it! Have never thought that it works in this way... Another problem is that how you can draw the conclusion "so \( 9^{70}≡1 \mod 100\)" from \(9^{10}≡1 \mod 100\) quickly?

    • 9 months ago
  78. Jack17
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    I raised both sides to the 7th power

    • 9 months ago
  79. RolyPoly
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    Ok, I didn't notice that, sorry!!

    • 9 months ago
  80. Jack17
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    If $$a\equiv b \text{ mod m}$$ and $$c\equiv d \text{ mod m}$$ then $$ac\equiv bd \text{ mod m}$$

    • 9 months ago
  81. RolyPoly
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    Thanks!!! It looks pretty good now :) Btw, if you want to type LaTeX in the same line as the words, use \( instead of \[ . Thanks again for your help :)

    • 9 months ago
  82. Jack17
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    ye no problem

    • 9 months ago
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