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Number theory Difficulty 4.9 AIME Prove it South Africa

Determine the last two digits of the product of the squares of all positive odd integers less than 20142014.

Solutions — 2

Solution 1

Since the product of the odd integers less than 20142014 contains 2525 as a factor, it is clearly divisible by 2525. Also, since it is odd, its last two digits have to be 2525 or 7575. So the product is either of the form 100n+25100n + 25 or of the form 100n+75100n + 75. In either case, the last two digits of the squared product (which is the same as the product of the squares) are 2525:
(100n+25)2=10000n2+5000n+625=100(100n2+50n+6)+25 (100n + 25)^2 = 10000n^2 + 5000n + 625 = 100(100n^2 + 50n + 6) + 25
or
(100n+75)2=10000n2+15000n+5625=100(100n2+150n+56)+25. (100n + 75)^2 = 10000n^2 + 15000n + 5625 = 100(100n^2 + 150n + 56) + 25.
In fact, we see that the last three digits have to be 625625.

Solution 2

Our number has 25=5225 = 5^2 as a factor, so its last two digits are 0000, 2525, 5050 or 7575. Moreover, it is the square of an odd number, which we can write as (2n+1)2=4n2+4n+1(2n+1)^2 = 4n^2 + 4n + 1. This means that the remainder upon division by 44 has to be 11. Now note that 100n+0100n + 0, 100n+25100n + 25, 100n+50100n + 50 and 100n+75100n + 75 leave a remainder of 00, 11, 22, 33 respectively when divided by 44. This means that the last two digits are in fact 2525.

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