F# string building

I want to do this C# code in F#

  string[] a = new string[5];
    string b = string.Empty;
    a[0] = "Line 1";
    a[2] = "Line 2";

    foreach (string c in a)
        b = c + Environment.NewLine;


Its a lot better to use the built-in String.Join method than rolling your own function based on repeated string concatting. Here's the code in F#:

open System
let a = [| "Line 1"; null; "Line 2"; null; null;|] 
let b = String.Join(Environment.NewLine, a)

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