I have moved this topic, as it is a better fit with discrete mathematics than number theory.
I am assuming (given the lack of bracketing symbols) that you are given to prove:
$$\prod_{j=1}^n\left(1+\frac{1}{n+j} \right)=2-\frac{1}{n+1}$$ where $$n\in\mathbb{N}$$.
The first thing we wish to do is demonstrate the base case $$P_1$$ is true:
$$\prod_{j=1}^1\left(1+\frac{1}{1+j} \right)=2-\frac{1}{1+1}$$
$$1+\frac{1}{1+1}=2-\frac{1}{1+1}$$
$$\frac{3}{2}=\frac{3}{2}$$
Thus, the base case is true.
Next, state the induction hypothesis $P_k$:
$$\prod_{j=1}^k\left(1+\frac{1}{k+j} \right)=2-\frac{1}{k+1}$$
Let's combine the two terms within the product:
$$\prod_{j=1}^k\left(\frac{k+j+1}{k+j} \right)=2-\frac{1}{k+1}$$
Let's pull out the first factor on the left.
$$\frac{k+2}{k+1}\prod_{j=2}^k\left(\frac{k+j+1}{k+j} \right)=2-\frac{1}{k+1}$$
This will allow us to re-index the product and replace $k$ with $k+1$:
$$\frac{k+2}{k+1}\prod_{j=1}^{k-1}\left(\frac{(k+1)+j+1}{(k+1)+j} \right)=\frac{2k+1}{k+1}$$
Next, multiply through by $$\frac{k+1}{k+2}$$:
$$\prod_{j=1}^{k-1}\left(\frac{(k+1)+j+1}{(k+1)+j} \right)=\frac{2k+1}{(k+1)+1}$$
Now, try as your induction step, multiplying by:
$$\prod_{j=k}^{k+1}\left(\frac{(k+1)+j+1}{(k+1)+j} \right)=\frac{2(k+1)+1}{2k+1}$$
What do you find?
Incidentally, there is an easier way to demonstrate the identity is true (if we hadn't been directed to use induction)...let's write the identity as:
$$\prod_{j=1}^n\left(\frac{n+j+1}{n+j} \right)=2-\frac{1}{n+1}$$
Now, we may choose to express this as:
$$\frac{\prod\limits_{j=1}^n\left(n+j+1 \right)}{\prod\limits_{j=1}^n\left(n+j \right)}=2-\frac{1}{n+1}$$
$$\frac{\prod\limits_{j=2}^{n+1}\left(n+j \right)}{(n+1)\prod\limits_{j=2}^{n}\left(n+j \right)}=2-\frac{1}{n+1}$$
$$\frac{(n+(n+1))\prod\limits_{j=2}^{n}\left(n+j \right)}{(n+1)\prod\limits_{j=2}^{n}\left(n+j \right)}=2-\frac{1}{n+1}$$
$$\frac{2n+1}{n+1}=2-\frac{1}{n+1}$$
$$\frac{2(n+1)-1}{n+1}=2-\frac{1}{n+1}$$
$$2-\frac{1}{n+1}=2-\frac{1}{n+1}$$