3.
    Sub Main()
        Dim K, p1, p2 As Double
        Dim z, f, n, i As Integer
        z = 6 : f = 9
        p1 = 1
        For n = 1 To 5
            p2 = 1
            For i = 1 To n
                p2 = p2 * i * i / (n * n)
            Next i
            p1 = p1 * (n - f * p2)
        Next n
        K = z - p2
        Console.WriteLine(p2)
        Console.ReadLine()
    End Sub
2.
    Sub Main()
        Dim a1, a2, a3, s As Double
        Dim i As Integer
        a1 = Math.Cos(2) / 2
        If a1 < 0 Then s = a1 Else s = 0
        a2 = Math.Sin(3) / 5
        For i = 3 To 17
            If a2 < 0 Then s = s + a2
            a3 = a2 - 4 * a1
            a1 = a2
            a2 = a3
        Next i
        Console.WriteLine("S= {0:f3}", s)
        Console.ReadLine()
    End Sub