excel中ppmt/pmt/ipmt的计算方式

参考来源： https://www.experts-exchange.com/articles/1948/A-Guide-to-the-PMT-FV-IPMT-and-PPMT-Functions.html

pmt计算公式：

 double pmt(double r, int nper, double pv, double fv, int type) {
         double pmt = r / (Math.pow(1 + r, nper) - 1)
                 * -(pv * Math.pow(1 + r, nper) + fv);

         return pmt;
     }

ppmt计算公式：

double ppmt(double r, int per, int nper, double pv, double fv, int type) {
        return pmt(r, nper, pv, fv, type) - ipmt(r, per, nper, pv, fv, type);
    }

ipmt计算公式

 double ipmt(double r, int per, int nper, double pv, double fv, int type) {

         double ipmt = fv(r, per - 1, pmt(r, nper, pv, fv, type), pv, type) * r;

         if (type == 1) ipmt /= (1 + r);

         return ipmt;
     }

fv计算公式：

double fv(double r, int nper, double c, double pv, int type) {
        double fv = -(c * (Math.pow(1 + r, nper) - 1) / r + pv
                * Math.pow(1 + r, nper));

        return fv;
    }