Help with Induction

Hey sci. I need some help with proof by induction. I have my conjecture, that f(n) = n/(2n+1), and my base case f(1) = 1/3, how can I prove that f(n+1) = (n+1)/(2n+3)?
Here's where I was heading: 1/3 + 2/5 + ... + n = n/(2n+1)
Where do I go from here?

I believe you need to backward substitute, then you'll hit the base case and discover the pattern.

A substance radioactively decays. On day 10 the substance had 120mg of mass. On Day 15 the substance had 11mfg of mass.

Model the mass of the element in this experiment as a function of time.

So the answer should be in the form  P(t)= C* e^(kt)  where:

t = time
C = population at time zero 0
k = rate of growth
P = population

dude
you created a formula for an element in the sequence
you are trying to find the sum

in the case of n = 2
1/3 + 2/5 != 2/5

f(n) = n / (2n + 1)
f(n + 1) = (n + 1) / (2n + 3)

n / (2n + 1) = (n - 1 + 1) / (2(n - 1) + 3)
n / (2n + 1) = n / (2n - 2 + 3)
n / (2n + 1) = n / (2n + 1)