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Number theory Difficulty 4.8 AIME Find the answer Croatia

Find the last two digits of the number (1!)2+(2!)2++(2018!)2(1!)^2 + (2!)^2 + \dots + (2018!)^2.

A number or a short expression. Spacing and $ signs are ignored.

Solution

For n10n \geq 10, n!n! is divisible by 1010, so (n!)2(n!)^2 is divisible by 100100. Therefore, only the terms (1!)2,(2!)2,,(9!)2(1!)^2, (2!)^2, \dots, (9!)^2 contribute to the last two digits.

Compute each term:

(1!)2=12=1(1!)^2 = 1^2 = 1

(2!)2=22=4(2!)^2 = 2^2 = 4

(3!)2=62=36(3!)^2 = 6^2 = 36

(4!)2=242=576(4!)^2 = 24^2 = 576

(5!)2=1202=14400(5!)^2 = 120^2 = 14400

(6!)2=7202=518400(6!)^2 = 720^2 = 518400

(7!)2=50402=25401600(7!)^2 = 5040^2 = 25401600

(8!)2=403202=1625702400(8!)^2 = 40320^2 = 1625702400

(9!)2=3628802=131681894400(9!)^2 = 362880^2 = 131681894400

Now, take the last two digits of each term:

(1!)2=1(1!)^2 = 1 (last two digits: 0101)

(2!)2=4(2!)^2 = 4 (last two digits: 0404)

(3!)2=36(3!)^2 = 36 (last two digits: 3636)

(4!)2=576(4!)^2 = 576 (last two digits: 7676)

(5!)2=14400(5!)^2 = 14400 (last two digits: 0000)

(6!)2=518400(6!)^2 = 518400 (last two digits: 0000)

(7!)2=25401600(7!)^2 = 25401600 (last two digits: 0000)

(8!)2=1625702400(8!)^2 = 1625702400 (last two digits: 0000)

(9!)2=131681894400(9!)^2 = 131681894400 (last two digits: 0000)

Sum the last two digits:
01+04+36+76+00+00+00+00+00=11701 + 04 + 36 + 76 + 00 + 00 + 00 + 00 + 00 = 117

The last two digits of 117117 are 1717.

Answer: 1717

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