Think!

x 1 , x 2 , x 3 , . . . . . . . . . , x 10 x_1 , x_2 , x_3 , ......... , x_{10} are integers , none of which are divisible by 3 . The remainder when x 1 2 + x 2 2 + x 3 2 + . . . . . . . + x 10 2 x^2_1 + x^2_2 + x^2_3 + ....... + x^2_{10} is divided by 3 is

This problem is a part of the sets - 3's & 4's & " N " for number theory

1 or 2 1 or 0 1 0,1 or 2

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2 solutions

Kalpok Guha
Mar 25, 2015

x 1 , x 2 , x 3 , . . . . . . . . . , x 10 x_1 , x_2 , x_3 , ......... , x_{10} these integers are in 3 k + 1 3k+1 or 3 k + 2 3k+2 form.

( 3 k + 1 ) 2 = 3 m + 1 (3k+1)^2=3m+1

( 3 k + 2 ) 2 = 3 n + 1 (3k+2)^2=3n+1

Thus the all ten integers are in 3 k + 1 3k+1 form

Their sum leaves remainder of 10 1 10*1 getting divided by 3.

The answer is 1 \boxed{1}

Matthew Burr
Oct 21, 2015

Given that none of x 1 , x 2 , x 3 , , x 10 x_{1},x_{2},x_{3},\ldots,x_{10} are divisible by 3, we know that for each i = 1 , 2 , 3 , , 10 i = 1,2,3,\ldots,10 , x i ≢ 0 ( m o d 3 ) x_{i}\not\equiv 0 \;\left(mod \; 3\right) . Then either x i 1 ( m o d 3 ) x_{i} \equiv 1 \;\left( mod \; 3 \right) or x i 2 ( m o d 3 ) x_{i} \equiv 2 \;\left(mod \; 3 \right) . If x i 1 ( m o d 3 ) x_{i} \equiv 1 \;\left(mod \; 3\right) , then x i 2 1 ( m o d 3 ) x_{i}^{2}\equiv1\;\left(mod\;3\right) . If instead x i 2 ( m o d 3 ) x_{i}\equiv2\;\left(mod\;3\right) , then x i 2 4 1 ( m o d 3 ) x_{i}^{2}\equiv4\equiv1\;\left(mod\;3\right) . In either case, x i 2 1 ( m o d 3 ) x_{i}^{2}\equiv1\;\left(mod\;3\right) , so x 1 2 + x 2 2 + x 3 2 + + x 10 2 10 1 ( m o d 3 ) x_{1}^{2}+x_{2}^{2}+x_{3}^{2}+\ldots+x_{10}^{2}\equiv10\equiv1\;\left(mod\;3\right) . Thus the remainder is 1.

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