Question

What is the sum of n natural numbers: 1+2+...+n?

Answer 1

1/2n(n+1)

Explanation:

Answer 2

S = 1/2 n (n+1)

Explanation:

1 + 2 + 3 + ... + n

Now write in reverse order:

n + n-1 + n-2 + ... + 1

Add together:

n+1 + n+1 + n+1 + ... + n+1

n (n+1)

Since this is double the sum:

S = 1/2 n (n+1)