Hey guys, I am trying to learn Mathematica and am attempting to compute pi using the Salamin and Brent equation. (Pic attached). The thing is, this equation is very different from the ones I attempted before as it doesn't have a typical summation.

To start this, I attempted to define a[0] = 1 and b[0]=1/sqrt[2] etc.However, this doesn't seem right as the letters turn green and I'm not able to assign that particular value to the aforementioned variables.

However, since I'm new to this kind of computing, I am just unsure about how to approach computing a sum like this. What are some codes I need to learn in order to be able to do this? I am blank as to how I must code these series and require guidance for where I can look at to learn how to code such formulae. Thank you very much!