getting error ==== Run Time Error"13"..
Mismatch..

hi everyone here is my code i m getting error [Run Time Error"13"..
Mismatch..] while exceuting this code

dont knw wats wrong

i m trying to caculate are under curve for "3" points if i enter (have
data set of around 1095 points)

j = 1 to 1092 then its working fine
but i want to mak this programe running for any data set
plz suggest to mak this code "more flexible to use"

Sub Button1_Click()

RowcountA = Cells(Rows.Count, "a").End(xlUp).Row


For j = 1 To RowcountA Step 3

x0 = Cells(j + 2, "a").Value
xn = Cells(j + 4, "a").Value

Cells(j + 2, "g").Value = x0
Cells(j + 2, "h").Value = xn

Cells(j + 2, "D").Select
ActiveCell.FormulaR1C1 = _
"=INDEX(LINEST(RC[-2]:R[2]C[-2],RC[-3]:R[2]C[-3]^{1,2}),1)"
Cells(j + 2, "E").Select
ActiveCell.FormulaR1C1 = _
"=INDEX(LINEST(RC[-3]:R[2]C[-3],RC[-4]:R[2]C[-4]^{1,2}),1,2)"
Cells(j + 2, "F").Select
ActiveCell.FormulaR1C1 = _
"=INDEX(LINEST(RC[-4]:R[2]C[-4],RC[-5]:R[2]C[-5]^{1,2}),1,3)"

p = Cells(j + 2, "d").Value
q = Cells(j + 2, "e").Value
r = Cells(j + 2, "f").Value

m = 100
n = m / 2
D = (xn - x0) / m

x = x0
y = p * x ^ 2 + q * x + r
Integ = y
Range("h4") = xn
x = xn
y = p * x ^ 2 + q * x + r
Integ = Integ + y

'[2] This loop adds terms 4*(y(1)+y(3)+ ... + y(2*n-1))
For i = 1 To 2 * n - 1 Step 2
x = x0 + i * D
y = p * x ^ 2 + q * x + r
Integ = Integ + 4 * y
Next i
'[3] This loop adds tersm 2*(y(2)+y(4)+...+y(2*n-2))
For i = 2 To 2 * n - 2 Step 2
x = x0 + i * D
y = p * x ^ 2 + q * x + r
Integ = Integ + 2 * y
Next i
'[4] The sum contained in "Integ" is now multiplied by Dx/3 to
finish calculation
Integ = Integ * D / 3
Cells(j + 2, "k") = Integ

' code to average depths

'Cells(j + 2, "L").Select
'ActiveCell.FormulaR1C1 = "=AVERAGE(RC[-9]:R[2]C[-9])"

' code to get volume

' Cells(j + 2, "m").Select
' ActiveCell.FormulaR1C1 = "=PRODUCT(RC[-2],RC[-1])"

Next j


End Sub

many thanx in advance