Don't understand

1/3 + 2/3^2 + 3/3^3 + 4/3^4 + 5/3^5 +............+ k/3^k =3/4

OK, CPhill or hectictar: use your brilliant algebraic knowledge to prove this. I can't !!.

I don't know how to prove this.....but maybe hectictar does......if not.....hang on for heureka........he's usually pretty good at these kind of things  !!!!

[ I always learn something from his answers ]

Don't wait up for me!! I never really learned how to do these "sum" things!!

Consider the infinite Geometric series $\displaystyle \frac{1}{x}+\frac{1}{x^{2}}+\frac{1}{x^{3}}+\dots$  .

Its sum to infinity (I think that there is a mistake in the question, there should be a plus sign and a series of dots after that last term), is $\displaystyle \frac{1}{x-1}$.

Equate the two, differentiate both sides, and then substitute x = 3.

Tiggsy