Let Find the remainder when is divided by 53.
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Firstly we can use the identity that ∑ i = 0 n ( i 2 ) = ( n 2 n ) . However the sum in the question begins at i = 1 , so we have to subtract one. So the question now is: Find ( 4 0 2 8 ! ) ÷ ( 2 0 1 4 ! ) ( 2 0 1 4 ! ) − 1 ( m o d 5 3 ) . Now, 2 0 1 4 ÷ 5 3 = 3 8 so 5 3 7 6 is the highest power of 53 which divides the denominator. 4 0 2 8 ÷ 5 3 = 7 6 , however one of these multiples of 53 is 5 3 2 so in fact 5 3 7 7 is the highest power of 53 which divides the numerator. So 5 3 ∣ ( 4 0 2 8 ! ) ÷ ( 2 0 1 4 ! ) ( 2 0 1 4 ! ) , and therefore ( 4 0 2 8 ! ) ÷ ( n ! ) ( n ! ) − 1 ≡ 0 − 1 ≡ 5 2 ( m o d 5 3 )