Analyze_This

Might be someway to use the DATA and STAT function to do this. This will give you the sum(x) and sum(x*y). You could then do sum(x*y)/sum(x) to get weighted average.

My top calculator tip is to use AOS mode. This forces the calculator to respect operator precedence rather than work things out as you type them in, so rather than faff about with brackets, you simply type things in as you'd write them down. In this mode you can do the weighted std-dev with cross term calc in a few seconds. In the other mode it took me 30-40 seconds due to the fiddling with brackets/mem slots.