或者写个这样的东西：

struct rat{
    ll x,y;
    void reduce(){
        if (y<0)
            x=-x,y=-y;
        ll gcd=__gcd(x<0?(-x):x,y);
        x/=gcd;
        y/=gcd;
    }
    rat(ll x=0,ll y=1){
        this->x=x;
        this->y=y;
        //printf("%lld\n",y);
        reduce();
        //print();
    }
    void print(){
        printf("%lld/%lld=%f\n",x,y,x*1.0/y);
    }
    rat operator *(const rat& rhs)const{
        return rat(rhs.x*x,rhs.y*y);
    }
    rat operator /(const rat& rhs)const{
        return rat(x*rhs.y,y*rhs.x);
    }
    rat operator -(const rat& rhs)const{
        return rat(x*rhs.y-y*rhs.x,y*rhs.y);
    }
    rat operator +(const rat& rhs)const{
        return rat(x*rhs.y+y*rhs.x,y*rhs.y);
    }
    int sgn(){
        if (!x)
            return 0;
        return x>0?1:-1;
    }
};

肉眼可见浮点误差吧...用log2()，换eps，扶老奶奶过马路...