mmult результат рассчитывается по VBA:
Option Explicit
Sub MatrixMultiplication()
Dim myArr1 As Variant
Dim myArr2 As Variant
Dim result As Variant
myArr1 = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
myArr2 = Application.WorksheetFunction.Transpose(myArr1)
Range("A2:A21") = myArr2
Range("B1:U1") = myArr1
result = Application.MMult(myArr2, myArr1)
Range("B2:U21") = result
'Range("B2:U21").FormulaArray = "=MMULT(A2:A21;B1:U1)"
'Range("B2:U21").FormulaArray = "=MMULT(RC[-1]:R[19]C[-1],R[-1]C:R[-1]C[19])"
End Sub