实现等本金还款计划生成

solve()

period    principal   interest      total      balance
     1      5000.00     700.00    5700.00    115000.00
     2      5000.00     670.83    5670.83    110000.00
     3      5000.00     641.67    5641.67    105000.00
     4      5000.00     612.50    5612.50    100000.00
     5      5000.00     583.33    5583.33     95000.00