#include <bits/stdc++.h>
using namespace std;
int main() {
double a, b, c;
cin >> a >> b >> c;
if(a==b)
{if (b!=c) {cout << "Ba so da nhap la do dai ba canh cua mot tam giac can";}
if(b==c) {cout << "Ba so da nhap la do dai ba canh cua mot tam giac deu";}}
if(b==c)
{if(a!=b) {cout << "Ba so da nhap la do dai ba canh cua mot tam giac can";}}
if(a==c)
{if(a!=b) {cout << "Ba so da nhap la do dai ba canh cua mot tam giac can";}}
if(a!=b)
{if(b!=c) {cout << "Ba so da nhap khong la do dai ba canh cua mot tam giac can";}}
return 0;
}

Chúc bn học tốt! (Bonus thêm trường hợp không là độ dài tam giác cân và là độ dài của tam giác đều nha!)

uses crt;

var a,b,c,p,s:real;

begin

clrscr;

readln(a,b,c);

if (a>0) and (b>0) and (c>0) and (a+b>c) and (a+c>b) and (b+c>a) then 

begin

p:=(a+b+c)/2;

s:=sqrt(p*(p-a)*(p-b)*(p-c));

writeln('Chu vi la: ',2*p:4:2);

writeln('Dien tich la: ',s:4:2);

end

else writeln('A B C khong phai la chieu dai ba canh cua mot tam giac');

readln;

end.

uses crt;

const fi='bai1.inp'

var f1:text;

a,b,c,cv,dt,p:real;

begin

clrscr;

assign(f1,fi); rewrite(f1);

write('Nhap a='); readln(a);

write('Nhap b='); readln(b);

write('Nhap c='); readln(c);

if (a>0) and (b>0) and (c>0) and (a+b>c) and (a+c>b) and (b+c>a) then 

begin

cv:=a+b+c;

p:=cv/2;

dt:=sqrt(p*(p-a)*(p-b)*(p-c));

writeln(f1,'Chu vi la: ',cv:4:2);

writeln(f1,'Dien tich la: ',dt:4:2);

end

else writeln(f1,'Day khong la ba canh trong mot tam giac');

close(f1);

readln;

end.

Var a,b,c: integer;
begin writeln('nhap ba so duong: ');Readln(a,b,c);
if (a+b>c) and (a+c>b) and (b+c>a) then writeln('3 so nay co the la do dai 3 canh cua 1 tam giac')
else writeln('3 so nay khong the la do dai 3 canh cua 1 tam giac');
End.

uses crt;
var a, b, c: real;
begin
clrscr;
 writeln('Kiem Tra Tam Giac');
    write('Nhap Ba Canh a, b, c:'); readln(a, b, c);
    if (a+b>c) and (a+c>b) and (b+c>a) then 
    begin
     writeln('Do La Ba Canh Tam Giac');
     if (a * a + b * b = c * c) or (a * a + c * c = b * b) or (b * b + c * c = a * a) then 
      writeln('Tam Giac Vuong')
     else if (a = b) or (a = c) or (b = c) then 
     begin
      if (a = b) and (b = c) then 
       writeln('Tam Giac Deu')
      else 
       writeln('Tam Giac Can');
     end
     else writeln('Tam Giac Thuong');
    end
    else write('Khong Phai Tam Giac');
    readln
end.

#include <bits/stdc++.h>

using namespace std;

double a,b,c;

int main()

{

cin>>a>>b>>c;

if (a>0 && b>0 && c>0 && a+b>c && a+c>b &&b+c>a) cout<<"phai";

else cout<<"ko phai";

return 0;

}

#include <bits/stdc++.h>

using namespace std;

double a,b,c;

int main()

{

cin>>a>>b>>c;

if (a>0 && b>0 && c>0 && a+b>c && a+c>b &&b+c>a) cout<<"phai";

else cout<<"ko phai";

return 0;

}

var a,b,c:integer;

begin

write('a = ');readln(a);

write('b = ');readln(b);

write('c = ');readln(c);

if (a+b>c) and (a+c>b) and (b+c>a) then

write('ba so do co the la do dai ba canh cua tam giac')

else write('ba so do khong the la do dai ba canh cua tam giac');

readln

end.

uses crt;

var a,b,c:real;

begin

clrscr;

repeat

write('Nhap a='); readln(a);

write('Nhap b='); readln(b);

write('Nhap c='); readln(c);

until (a>0) and (b>0) and (c>0);

if (a+b>c) and (a+c>b) and (b+c>a) then 

begin

writeln('La do dai ba canh cua tam giac');

writeln('Chu vi tam giac la: ',a+b+c:4:2);

end

else writeln('Khong phai la do dai 3 canh cua tam giac');

readln;

end.