Cộng, trừ, nhân, chia phân số

Đề bài: làm phép cộng, trừ, nhân, chia phân số

var
        a,b,c,d:longint;

function ucln(a,b:longint):longint;
var     r:longint;
begin
        while a mod b <>0 do
                begin
                        r:=a mod b;
                        a:=b;
                        b:=r;
                end;
        exit(b);
end;

procedure cong(a,b,c,d:longint);
var     tu, mau,UC:longint;
begin
        tu:=a*d+b*c;
        mau:=b*d;
        UC:=ucln(tu,mau);
        tu:=tu div uc;
        mau:=mau div uc;
        if mau=1 then
                writeln(a,'/',b,' + ',c,'/',d,' = ',tu)
        else
                writeln(a,'/',b,' + ',c,'/',d,' = ',tu,'/',mau);
end;

procedure tru(a,b,c,d:longint);
var     tu, mau,UC:longint;
begin
        tu:=a*d-b*c;
        mau:=b*d;
        UC:=ucln(tu,mau);
        tu:=tu div uc;
        mau:=mau div uc;
        if mau=1 then
                writeln(a,'/',b,' - ',c,'/',d,' = ',tu)
        else
                writeln(a,'/',b,' - ',c,'/',d,' = ',tu,'/',mau);
end;

procedure nhan(a,b,c,d:longint);
var     tu, mau,UC:longint;
begin
        tu:=a*c;
        mau:=b*d;
        UC:=ucln(tu,mau);
        if tu=0 then
                begin
                        writeln(a,'/',b,' * ',c,'/',d,' = ',0);
                        exit;
                end;
        tu:=tu div uc;
        mau:=mau div uc;
        if mau=1 then
                writeln(a,'/',b,' * ',c,'/',d,' = ',tu)
        else
                writeln(a,'/',b,' * ',c,'/',d,' = ',tu,'/',mau);
end;

procedure chia(a,b,c,d:longint);
var     tu, mau,UC:longint;
begin
        tu:=a*d;
        mau:=b*c;
        UC:=ucln(tu,mau);
        if tu=0 then
                begin
                        writeln(a,'/',b,' : ',c,'/',d,' = ',0);
                        exit;
                end;
        tu:=tu div uc;
        mau:=mau div uc;
        if mau=1 then
                writeln(a,'/',b,' : ',c,'/',d,' = ',tu)
        else
                writeln(a,'/',b,' : ',c,'/',d,' = ',tu,'/',mau);
end;

begin
        writeln('Nhap Tu So phan so thu Nhat: ');read(a);
        writeln('Nhap Tu So phan so thu Nhat: ');read(b);
        writeln('Nhap Tu So phan so thu Hai: ');read(c);
        writeln('Nhap Tu So phan so thu Hai: ');read(d);
        cong(a,b,c,d);
        tru(a,b,c,d);
        nhan(a,b,c,d);
        chia(a,b,c,d);
        readln;
        readln;
end.