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Printjmc
number theory senior
Problem
What is the sum of the last two digits of this portion of the Fibonacci Factorial Series: ?
Solution
This expression , is the number you get by multiplying by by by and so on, all the way down to . So . Notice that ends in a since it has a factor of (there is a and a in it list of factors) and that has to end in two zeroes since it has a factor of , and which is really a factor of . Since any factorial greater than (such as or ) includes all of the factors of , the last two digits of , , and so on are zeroes. These terms, therefore will not affect the last two digits of the sum of the Fibonacci factorial series. To find the last two digits, you only need to find the last two digits of each of the terms of . We do not need to calculate , only to find its last two digits. Starting with , we can work our way to , using only the last two digits of each value along the way. We know , so use when finding , which will bring us to or . Therefore, the last two digits of are from or . Finally is or finally . The last two digits of the entire series will come from . Therefore, the sum of the last two digits is .
Final answer
5