Verified answer

1) u1 = 3 * 1 = 3

u2 = 3 * 2 = 6

u3 = 3 * 3 = 9

2) The sum of the first k natural numbers is 1 + 2 + 3 + ... + k = k(k + 1)/2

The sum of the first 20 terms of the sequence is

3*1 + 3*2 + 3*3 + ... + 3*20 = 3*(1 + 2 + 3 + ... + 20) = 3 * 20 * 21 / 2 = 630

The sum of the first 100 terms of the sequence is

3*1 + 3*2 + 3*3 + ... + 3*100 = 3*(1 + 2 + 3 + ... + 100) = 3 * 100 * 101 / 2 = 5050

I don't fully understand the meaning of (n = 1) and (n = 21).