#include <bits/stdc++.h>

using namespace std;

double a,s;

int main()

{

cin>>a;

s=1/2*a*a;

cout<<fixed<<setprecision(2)<<s;

return 0;

}

var a,b,s:integer;

c:word;

begin

write('nhap avà b ');

read(a,b);

s:=a*b*1/2;

c:=(a+b)*2;

writeln('dien tich la '; s );

write('chu vi la 'c);

readln

end

program CVDT;

uses crt;

var a,b,c,P,S;

begin

Writeln('Nhap a va b');

Readln(a,b);

c:= a*a +b*b; S:= (a*b)/2;

Writeln('Dien tich tam giac vuong la ',S);

writeln('Chu vi tam giac vuong la ', SQRT(c);

Readln;

end.

Câu 1: 

uses crt;

var a,s,p:real;

begin

clrscr;

repeat

write('Nhap do dai canh hinh vuong:'); readln(a);

until a>0;

p:=4*a;

s:=sqr(a);

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

writeln('Dien tich hinh vuong la: ',s);

readln;

end.

Câu 2: 

uses crt;

var a,b,t,s,h:real;

begin

clrscr;

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

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

t:=a+b;

h:=a-b;

s:=a*b;

writeln('Tong la: ',t:4:2);

writeln('Hieu la: ',h:4:2);

writeln('Tich la: ',s:4:2);

readln;

end.

Câu 3: 

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 writeln('Day la ba canh trong mot tam giac')

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

readln;

end.

ta có độ dài AB là : \(\left(17+7\right):2=12cm\)

độ dài AC là : \(12-7=5cm\)

độ dài cạnh BC là : \(BC=\sqrt{12^2+5^2}=13cm\)

Chu vi tam giác ABC là : \(AB+BC+AC=12+5+13=30cm\)

DIện tích tam giác ABC là : \(AB\times\frac{AC}{2}=12\times\frac{5}{2}=30cm^2\)

Program dung;

uses crt;

var s,a,b: real;

begin

clrscr;

write('Nhap a; b : '); readln(a,b);

s:=(a*b)/2;

writeln('Dien tich tam giac vuong la: ',s:1:2);

readln;

end.

Uses crt;

var i,n,y: integer;

begin clrscr;

readln(n);

for i:=1 to n do s:=i*i;

Writeln(s);

readln;

end.

Program HOC24;

var i,n: integer;

s: longint;

begin

write('Nhap n: '); readln(n);

s:=0;

for i:=1 to n do s:=s+sqr(i);

write('S= ' ,s);

readln

end.