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joemama142000
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joemama142000
What is the remainder when x^2+y^2 is divided by 5, where x and y are positive integer?
1). When x+y is divided by 5, the remainder is 1.
2). When x-y is divided by 5, the remainder is 2.


Two ways to solve:

A) x+y = 1( mod 5) ---> (x+y)^2 = 1 (mod 5)
x-y = 2 (mod 5) ----> (x-y)^2 = 4 ( mod 5)
---> (x-y)^2 + (x+y)^2 = 5 ( or 0 ) (mod 5) ---> 2(x^2+ y^2) = 0 ( mod 5) ---> x^2 + y^2 divided by 5 has remainder of 0

B) from 1 and 2: x+y= 1( mod 5) and x-y= 2( mod 5)
Add them together: 2x= 3 ( mod 5) --> x= 4 (mod 5) --> y= 2( mod 5)
--> x^2= 1 (mod 5) and y^2= 4 ( mod 5) --> x^2 + y^2= 0 (mod 5)

C it is.
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joemama142000
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laxie, can you explain what is mod? I am confused by your answer. thanks :?
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laxieqv
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joemama142000
laxie, can you explain what is mod? I am confused by your answer. thanks :?


Mod= modulus.
For example if x divided by 7 has remainder of 5, we'll write x this way:
x= 5 (mod 7)

Btw, thank you for your PM, buddy!! :)
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How about this -

x^2+y^2 = (x+y)^2 + (x-y)^2 / 2

So we need to know both 1 and 2 to know the remainder.

Hence C.

- Vipin
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statement 1 gives us following possible pairs of last digits

0 6
5 1
4 2
3 3

0 1
5 6
7 4
9 2
8 3

NOT suff

statement 2 gives us

8 1
9 2
0 3

3 1
4 2
5 3
6 4
7 5
8 6
9 7
2 0


NOT suff

using statement 1 and 2 together


4 2 pair is unique common So C



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