a borrower has a mortgage that calls for level annual payments of 1 at the end of each year for 20 years. at the time of the seventh regular payment an additional payment is made equal to the amount of principal that according to the original amortization schedule would have been repaid by the eight regular payment. if payments of 1 continue to be made at the end of the eighth and succeeding years until the mortgage is fully repaid, show that the amount saved in interest payments over the full term of the mortgage is 1-v^13