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Algebra Level pending

What will be the remainder if 6 7 67 + 67 67^{67}+67 is divided by 68 68 ?


The answer is 66.

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3 solutions

Datu Oen
Mar 20, 2014

Since 67 1 ( m o d 68 ) 67 \equiv -1 \pmod {68}

Then 6 7 67 ( 1 ) 67 ( m o d 68 ) 67^{67} \equiv (-1)^{67} \pmod {68}

( 1 ) 67 1 ( m o d 68 ) (-1)^{67} \equiv -1 \pmod {68}

Therefore, 6 7 67 + 67 1 + ( 1 ) ( m o d 68 ) 2 ( m o d 68 ) 67^{67} + 67 \equiv -1 + (-1) \pmod {68} \equiv -2 \pmod{68} .

Finally, 2 66 ( m o d 68 ) -2 \equiv 66 \pmod{68}

Pratik Ranjan
May 1, 2014

67^67+67 can be wriiten as (68-1)^67+67 .. pllying bnomila exapnasion for the first term we have 67c068^0(-1)^67+67c168^1(-1)^66+67c268^2(-1)65...............................................67c6768^67 +67 -1+68(67c168^0(-1)^66+67c268^1(-1)65...........................67c6768^66) +67 66+68(67c168^0(-1)^66+67c268^1(-1)65...........................67c6768^66) which is of the form r+68q .... where r is remainder and q is qoutient .. hence r=66

67^2 when divided by 68 will leave a remainder 1. So 67^64 when divided by 68 will leave a remainder 1. And 67^3 when divided by 68 will leave a remainder 67. Therefore the remainder when 67^67 will be divided by 68 will be 67. Therefore when 67+67=134 when divided by 68 the remainder will be 66. That is the answer. :)

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