What is the remainder when 1 5 + 2 5 + 3 5 + ⋯ + 9 9 5 + 1 0 0 5 is divided by 4?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
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 ) . Now, we can write 1 5 + 2 5 + … + ( 9 9 ) 5 + ( 1 0 0 ) 5 = ( 1 0 0 ) 5 + ( 5 0 ) 5 + i = 1 ∑ 4 9 [ i 5 + ( 1 0 0 − i ) 5 ] = ( 1 0 0 ) 5 + ( 5 0 ) 5 + i = 1 ∑ 4 9 1 0 0 × [ i 4 − i 3 ( 1 0 0 − i ) + i 2 ( 1 0 0 − i ) 2 − i ( 1 0 0 − i ) 3 + ( 1 0 0 − i ) 4 ] . Clearly each of the terms is a multiple of 100.
Since 100 is divisible by 4, the remainder is 0 .
∑ i = 1 1 0 0 i 5 = 2 5 × ∑ i = 1 4 i 5 = 2 5 × ( 1 5 + 3 5 ) = 2 5 × ( 1 + 3 ) = 0 ( m o d 4 )
Problem Loading...
Note Loading...
Set Loading...
We could just ignore the even terms ( 2 5 , 4 5 , . . . etc.)
If n ≡ 1 ( m o d 2 ) , then n ≡ n 5 ( m o d 4 ) .
So we can consider 1 + 3 + 5 + . . . + 9 7 + 9 9 = 5 0 2 which is divisible by 4.