Hello,

I assume that you know that this proof has to be done in 3 steps:

S1: n = 1: . S1 is true.

S2: We assume that 3 + 6 + 9 + ... + 3n = 3n(n+1)/2 is true.

S3: Show that the equation is true for n+1:

Thus 3 + 6 + 9 + ... + 3n = 3n(n+1)/2 is true for n+1 and therefore it is true for all