You want to determine the monthly
payment it takes to pay off a loan (i.e.,
`amortize`

the loan). Alternatively, you want to
know how much money you can withdraw from a retirement account on a
periodic basis.

Calculate the payments using the custom ```
Math.PMT(
)
```

function.

Suppose you want to borrow $100,000 from a bank, and you need to know
the required monthly payment to pay back the loan. The answer
requires the bank to calculate how much money is needed to pay off
the principal and the interest that accrues before the loan is
finally paid off in full. The calculation is equivalent to the one
needed to calculate how much money you can take out monthly from a
retirement account. However, in the case of a so-called
`annuity`

, you start with the lump sum and extract
a series of periodic payments. The math still answers the question,
“Given an initial pile of money and the prevailing
interest rate, how much money can I take out (or must I pay back)
each month before the balance is zero?”

The following custom `Math.PMT( )`

function
returns the same value as the one obtained in Microsoft Excel using
the Insert → Function → Financial
→ PMT formula. You can add this to your
`Math.as`

file for easy inclusion in other
projects.

Math.PMT = function (i, n, PV) { //= initial savings deposit or loan amount (present value) //`PV`

= periodic interest rate //`i`

= number of payment periods //`n`

= periodic payment // Calculate ...`PMT`

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