Variable range in vba function formula

Hello,
If i want to do this formula

((1/c1)^2+(1/c2)^2 .............................. + (1/c)^2 )^(-0.5)

for variable range by ( vba function) or excel formula

note:

i can use this way but i need to use variable range

Public Function Add(number1 As Double, number2 As Double)
Add = (1 / (number1) ^ 2 + 1 / (number2) ^ 2) ^ (-0.5)
End Function