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Basic Congruences Confusion

by mickles
Tags: basic, confusion, congruences
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mickles
#1
Nov12-12, 04:52 PM
P: 8
Hi, this is not a homework problem, i just have a hard time following the sequence of this

In the book , it shows a couple examples

(=== is the triple equal sign)

1) 26=19+7===3+7===10(mod8)
2) 15=19-4===3-4=-1(mod8)
3)38 = 19*2===3*2=6(mod8)
4) 7===2(mod5), 343=7^3===2^3=8(mod5)

I understand mod8 and whatnot, just how does the book go from 19+7 to 3+7, and from 19-4 to 3-4. I just don't get how 19 and 3 are logically connected

I see how 15===-1(mod8) and 26===10(mod8) and 38=6(mod8).

Any help understanding is appreciated
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Petek
#2
Nov12-12, 05:39 PM
Petek's Avatar
P: 361
The examples you cited are using the following property of congruences: If [itex]a \equiv b \pmod{m}[/itex], then [itex] a + c \equiv b + c \pmod{m}[/itex]. So, in your first example, let a = 19, b = 3, c = 7 and m = 8.

Does that help?
ramsey2879
#3
Nov12-12, 05:46 PM
P: 894
Quote Quote by mickles View Post
Hi, this is not a homework problem, i just have a hard time following the sequence of this

In the book , it shows a couple examples

(=== is the triple equal sign)

1) 26=19+7===3+7===10(mod8)
2) 15=19-4===3-4=-1(mod8)
3)38 = 19*2===3*2=6(mod8)
4) 7===2(mod5), 343=7^3===2^3=8(mod5)

I understand mod8 and whatnot, just how does the book go from 19+7 to 3+7, and from 19-4 to 3-4. I just don't get how 19 and 3 are logically connected

I see how 15===-1(mod8) and 26===10(mod8) and 38=6(mod8).

Any help understanding is appreciated
Any number A === A - N Mod N Thus 19 === 11 === 3 Mod 8, Therefore 19+7 === 3+7 and 19-4 === 3-4.

mickles
#4
Nov12-12, 05:48 PM
P: 8
Basic Congruences Confusion

Quote Quote by Petek View Post
The examples you cited are using the following property of congruences: If [itex]a \equiv b \pmod{m}[/itex], then [itex] a + c \equiv b + c \pmod{m}[/itex]. So, in your first example, let a = 19, b = 3, c = 7 and m = 8.

Does that help?
Yes that makes a lot more sense now with a,b,c, and m after looking at the theorem.

Thanks for you help
mickles
#5
Nov12-12, 05:50 PM
P: 8
Quote Quote by ramsey2879 View Post
Any number A === A - N Mod N Thus 19 === 11 === 3 Mod 8, Therefore 19+7 === 3+7 and 19-4 === 3-4.
thank you this also helped


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