Question

The unit digit of 1 + 2 + 3 + 4 + ... + 100 is
(A) 3
(B) 1
(C) 2
D) 0​

Answer 1

Answer:

1! + 2! + 3! +4! + 5! =6! +…..+ 100!

= 1 + 2 + 6 + 24 + 120 + 6*120 + 7*6*120 +… +100!

It can be seen that after 4!, all terms in the series are divisible by 10. Therefore, they all have a units digit of 0, and so they will not affect the units digit of the sum of the series.

Therefore, the units digit of the sum of the series is determined by the units digit of the first 4 terms:

1 + 2 + 6 + 24 = 33, which has a units digit of 3.

Therefore, the units digits of 1! + 2! + 3! +4! + 5! =6! +…..+ 100! is 3.

Here is a Python (2.7) solution:

Step-by-step explanation:

OPTION, (A) 3

Answer 2

Answer:

0

Step-by-step explanation:

S = 1 + 2 + 3 + 4 + + 100

S = 100×101/2 = 5050