Euler.pas

uses math;

const
    x0 = 0;
    y0 = 1;
    b = 1;
    h = 0.0001;

function f(x, y : real) : real;
begin
    f := x*y;
end;

function Y_(x : real) : real;
begin
    Y_ := exp(x*x/2);
end;

var
    i, n : integer;
    x, y : real;

begin
    n := trunc((b - x0) / h);

    x := x0;
    y := y0;
    for i := 1 to n do begin
        y += f(x, y) * h;
        x := x0 + i*h;
    end;

    writeln(abs(Y_(b) - y):0:6);
end.