The ANNUITY function (annuity immediate) returns a numeric value that approximates the ratio of an annuity paid at the end of each period for the number of periods specified by argument-2 to an initial investment of one. Interest is earned at the rate specified by argument-1 and is applied at the end of the period, before the payment. The type of this function is numeric.
1 / argument-2
argument-1 / (1 - (1 + argument-1) ** (- argument-2))