SOLUTION: s= 3 + 3 ^ 2 + 3 ^ 3 +***+3^ z If s is the sum of the series given above, for which of the following values of n will s be divisible by 10?

Question 1201932: s= 3 + 3 ^ 2 + 3 ^ 3 +***+3^ z
If s is the sum of the series given above, for which of the following values of n will s be divisible by 10?
Answer by greenestamps(13200) (Show Source): You can put this solution on YOUR website!

s= 3 + 3 ^ 2 + 3 ^ 3 +...+3^ z
If s is the sum of the series given above, for which of the following values of z will s be divisible by 10?

The sum is divisible by 10 if the units digit of the sum is 0.

z=1: units digit of the sum is 3
z=2: units digit of the sum is 3+9 = 12 --> 2
z=3: units digit of the sum is 3+9+7 = 19 --> 9
z=4: units digit of the sum is 3+9+7+1 = 20 --> 0

The units digits of the sums for larger values of z will follow the same pattern, so the units digit of the sum will be 0 for any value of z that is a multiple of 4.

ANSWER: any multiple of 4

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