PPMT(rate,per,nper,pv,fv,type)
rate: effective annual interest rate
per: period number
nper: number of periods
pv: present value
fv: future value
type: payment type
PPMT calculates the principal part of an annuity's payment for period number per.
If type is 0, the default, payment is at the end of each period. If type is 1, payment is at the beginning of each period.
IPMT.