The ‘real’ math behind Excel formulas

Is there a way, within Excel or somewhere, to find the true mathematical calculation or equation that Excel is using when the formula is entered with a function word?

For example, if I use the function formula word =AVERAGE(A1:A5), I would like to find the ‘real’ equation like =(A1+A2+A3+A4+A5)/5

What I’m especially interested is in the actual mathematical formula for the =PMT and =FORECAST function words.