About remainder question

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About remainder question

by neilcao » Tue Feb 08, 2011 8:12 pm
Maybe a poor example, some questions like this say: (7^243)+(7^22)-5 divided by 7, what is the remainder?

How to solve this kind of problem?


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by BarryLi » Tue Feb 08, 2011 9:19 pm
I think it's important to look at the terms and how they fit to solve this question.

(7^243)+(7^22)-5 divided by 7 = [ (7^243) + (7^22) - 5 ] / 7

This expression can be broken down further by dividing 7 into each term.

(7^243)/7 + (7^22)/7 - 5/7

The 1/7 term seemlessly goes into the 7^243 and 7^22 term, because 1/7 = 7^(-1), i.e. two numbers with the same base (7) are being multiplied. This means there is no remainder.

The - 5 term isn't so fortunate. 7 is too big to be to divided into 5, so 5 is the remainder.

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by Anurag@Gurome » Tue Feb 08, 2011 11:30 pm
neilcao wrote:(7^243)+(7^22)-5 divided by 7, what is the remainder?
(7^243) + (7^22) - 5 = (Some multiple of 7) - 5
Now any integer less than a multiple of 7 by 5, when divided by 7 will leave a remainder of (7 - 5) = 2

Because, 7k - 5 = 7(k - 1) + 7 - 5 = 7(k - 1) + 2
Where k is any positive integer.

Note: If the expression was (7^243) + (7^22) + 5, then the remainder would've been 5.
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by BarryLi » Wed Feb 09, 2011 11:14 pm
Wow how embarrassing, thanks for the correction Anurag@Gurome