annuity_PV = function(A, n, r){
    (A/r) * ( 1 - 1/(1+r)^n )
}

annuity_payment = function(P, n, r){
    numer = r * (1+r)^n * P
    denom = (1+r)^n - 1 
    A = numer/denom 
}