Почему во 2 и 3 задачи решётки? зачем они

int a = 3, b = 1;
double s = 0, t = 1;
for (a = 3; a <= 8; a++)
{
s = 0;
for (b = 1; b <= 5; b++)
s = s + Math.Pow(Math.Cos(a),2)*b;
t = t * s;
}
Console.WriteLine("t = {0:#.##### }", t);
Console.ReadKey();

double a = 2.5, t = 0, x=2.1, n;
for (n = 1; n <= 12; n++)
t = t + Math.Sin(a)*n;
t = x * Math.Sin(a) * t;
Console.WriteLine("Сумма членов ряда = {0:#.###}", t);
Console.ReadKey();

class Program
{
static void Main(string[] args)
{
double z, a=2, b=5, x = 0;
while (x <= 5)
{
if (x <= 3) z = x+Math.Tan(x);
else z = a*Math.Pow(x,2) + b*x;
Console.WriteLine(" x = {0,2:f} | z = {1,3:f}", x, z);
x = x + 0.5;
}
Console.ReadKey();

в 1 и 2