Final answer:
To find the perimeter of a triangle, use the formula P = a + b + c, where a, b, and c are the lengths of the sides.
Step-by-step explanation:
To find the perimeter of a triangle, you need to know the lengths of all three sides. Here's a Pascal program that calculates the perimeter:
 
program PerimeterTriangle;
var
 Side1, Side2, Side3, Perimeter: real;
begin
 writeln('Enter length of side 1: ');
 readln(Side1);
 writeln('Enter length of side 2: ');
 readln(Side2);
 writeln('Enter length of side 3: ');
 readln(Side3);
 Perimeter := Side1 + Side2 + Side3;
 writeln('Perimeter of the triangle: ', Perimeter);
end.