VBA Trend function with variable rows

[uti1301]

I have 17 columns A to R.
I want to use the TREND function to enter values in column R, starting at row 3.

I have the following code successfully for rows 3 to 39:

sub test()
dim lastforecastmonth as long
lastforecastmonth=cells(rows.count, "A").end(xlup).row
range("R").select
for I=0 to lastforecastmonth-3
activecell.formulaR1C1= "=trend(R3C2:R39C2, R3C5:R39C17, RC[-13]:RC[-17]"
next I
end sub

My problem is that the number of rows can vary.

I would be very grateful if someone could tell me how to do this

Thanks in advance for any help

 

[uti1301]

sorry, line 6 of code should be:
activecell.offset(I,0).formulaR1C1= "trend(R3C2:R39C2, R3C5:R39C17, RC[-13]:RC[-17])"

 

[uti1301]

sorry again, line 6 of code should be:
activecell.offset(I,0).formulaR1C1= "=trend(R3C2:R39C2, R3C5:R39C17, RC[-13]:RC[-1])"​