Sub func()
N = Cells(2, 1)
a = Cells(2, 2)

Sum1 = 0
fa = 1
For i = 0 To N
fa = fa * (a + i)
Sum1 = Sum1 + 1 / fa
Next i
Cells(4, 1) = Sum1

Sum2 = 1 / a
For i = 1 To N
Sum2 = Sum2 + 1 / a ^ (2 * i)
Next i
Cells(6, 1) = Sum2

Sum3 = a
For i = 1 To N
Sum3 = Sum3 * (a - i * N)
Next i
Cells(8, 1) = Sum3
End Sub