I haven’t done jc maths in a while, so my last statement might not be how your school requires you to explain your step. But generally for mathematical induction, you show the base case is true (here, n=1 is the base case since n is any positive integer), then you assume n=k is true and prove for n=k+1 (you will most of the time require the equation from n=k) here. You can observe for my first step of proving n=k+1, I split the equation to 2 portions so I can make use of my n=k equation :)

Hope this helps :)