Yes it deemed possible!

What is the remainder when 1 5 + 2 5 + 3 5 + + 9 9 5 + 10 0 5 1^5 +2^5 +3^5 + \cdots + 99^5 + 100^5 is divided by 4?


The answer is 0.

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3 solutions

X X
Sep 15, 2018

We could just ignore the even terms ( 2 5 , 4 5 , . . . 2^5,4^5,... etc.)

If n 1 ( m o d 2 ) n\equiv 1\pmod2 , then n n 5 ( m o d 4 ) n\equiv n^5\pmod4 .

So we can consider 1 + 3 + 5 + . . . + 97 + 99 = 5 0 2 1+3+5+...+97+99=50^2 which is divisible by 4.

Syed Hamza Khalid
Sep 15, 2018

The sum is actually divisible by 100. Just observe that a 5 + b 5 = ( a + b ) ( a 4 a 3 b + a 2 b 2 a b 3 + b 4 ) . a^5 + b^5 = (a+b) (a^4 - a^3 b + a^2 b^2 - ab^3 +b^4). Now, we can write 1 5 + 2 5 + + ( 99 ) 5 + ( 100 ) 5 = ( 100 ) 5 + ( 50 ) 5 + i = 1 49 [ i 5 + ( 100 i ) 5 ] = ( 100 ) 5 + ( 50 ) 5 + i = 1 49 100 × [ i 4 i 3 ( 100 i ) + i 2 ( 100 i ) 2 i ( 100 i ) 3 + ( 100 i ) 4 ] . \begin{aligned} & 1^5 + 2^5 + \dotso + (99)^5 + (100)^5 \\ & = (100)^5 + (50)^5 + \sum_{i=1}^{49} [ i^5 + (100 - i)^5] \\ & = (100)^5 + (50)^5 + \sum_{i=1}^{49} 100 \times [ i^4 - i^3(100-i) + i^2 (100-i)^2 - i(100 - i)^3 + (100-i)^4] .\end{aligned} Clearly each of the terms is a multiple of 100.

Since 100 is divisible by 4, the remainder is 0 . \large \color{#3D99F6} \boxed{0} .

i = 1 100 i 5 = 25 × i = 1 4 i 5 = 25 × ( 1 5 + 3 5 ) = 25 × ( 1 + 3 ) = 0 ( m o d 4 ) \sum_{i=1}^{100}i^5=25 \times \sum_{i=1}^{4}i^5= 25 \times (1^5+3^5)= 25 \times (1+3)= 0 \ (mod 4)

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