using excel array functions on part of array

say I have arrays ArrA and ArrB, of lengths x and y.

I know I can write, say,

Application.WorksheetFunction.SumProduct(ArrA,ArrB)

to get the sumproduct off the two arrays. however, say i want to sumproduct of just the first z elements of the arrays. is there a neat way of getting this?

try something like this

Sub test()
Dim arra As Range, arrb As Range, rnga As Range, rngb As Range
Dim j As Double
Set arra = Range(Range("a1"), Range("a1").End(xlDown))
Set arrb = Range(Range("b1"), Range("B1").End(xlDown))


Set rnga = arra.Resize(3, 1)
Set rngb = arrb.Resize(3, 1)

MsgBox rnga.Address
j = WorksheetFunction.SumProduct(rnga, rngb)
MsgBox j
End Sub