public Rat QuotRat(Rat r) => new Rat(this.num * r.den, this.den * r.num);
public Rat SubRat(Rat r) => new Rat(this.num * r.den - r.num * this.den, this.den * r.den);
public Rat MultRat(Rat r) => new Rat(this.num * r.num, this.den * r.den);
public Rat SumRat(Rat r) => new Rat(this.num * r.den + r.num * this.den, this.den * r.den);