I am bit confused while using the method "formula"

please see the code statements and result in running each of the code
independently
===============
Range("c1") = Range("a1") * Range("b1") '----ok
range("c1").Formula="=range("a1")*range("b1")"----results in syntax error
exected end of statement at a1
Range("c1").Formula = "=$A$1*$B$1" - -------ok
Range("c1").Formula = "=cells(1,1)*cells(1,2)"---returns#NAME#
Range("c1").Formula = "=cells(0,[-1])*cells(0,[-2])"---appln defined
orobject defined error
Range("c1").FormulaR1C1 = "=rc1*rc2" - --------ok
ActiveCell = ActiveCell.Offset(0, -2) * ActiveCell.Offset(0, -1) - ---ok
ActiveCell.Formula = "=ActiveCell.Offset(0, -2) *
ActiveCell.Offset(0, -1)"--returns #NAME#
===============

request for some logical ideas in using "formula"