Find the remainder when is divided by 100.
Solution
For convenience we define and for odd positive integers . Then the question asks for the last two digits of .
Since is odd, we have .
As and the units digits of the powers of 7 (also the powers of 17) follows the pattern 7, 9, 3, 1, 7, 9, 3, 1, which repeats itself every four terms, we conclude that the units digit of is the same as that of , which is 3.
Finally, if we look at the last two digits of the powers of 19, we will see the pattern 19, 61, 59, 21, 99, 81, 39, 41, 79, 01, 19, 61, which repeats itself every 10 terms. As has units digit 3, the last two digits of are the same as those of , which are 59.
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