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borrowing money
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2022-08-26
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BORROWING MONEY
Now suppose you wish to take out a
loan. Here are the things you must
consider.
P - The amount of the loan. The
higher the loan, the higher the
monthly payment.
i - The interest rate (PER PERIOD).
This is considered the most
important item to shop for. The
higher the interest rate, the
higher the payment.
n - The number of payments.
R - The amount of each payment.
The important formula which relates
the four variables P, i, n, and R is
derived from the following sensible
approach to lending.
A loan of amount P, interest rate i,
to be repaid in n monthly equal
payments is an annuity. As such, it
has an amount at the end of the term.
That is, the sum of the compound
amounts at the end of the term. That
amount is
n
R((1+i) -1)
S = ----------
i
If that same amount had not been
lent to you, but rather invested in an
account at the same interest rate, i
for n months, the lender would have
n
S = P(1+i)
in his account at the end of the term
It is reasonable then, that the lender
shouldn't lose money lending to you.
The equation then is that the amount
of the annuity at the end of the term
is equal to the amount that would have
been in his account if he had not lent
it to you. Thus
n
n R((1+i) -1)
P(1+i) = ----------
i
is the fundamental equation relating
all four variables.
The equation can be solved for P, R,
and n, but i can only be approximated.
-n
R(1-(1+i) )
P = -----------
i
iP
R = -----------
-n
1 - (1+i)
iP
log(1 - ----)
R
n = - -------------
log (1+i)
Remember, the interest rate i in the
above formulas is the interest rate
PER PERIOD. For instance, if you make
monthly payments at an APR of 12%,
then i is 1%, NOT 12%.
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