Dimension member property in dimension formula

I have to use time dimension member property in dimension formula but unable to use. None of the below syntax work and gives error. What am i missing.

I have to use act1/[time].currentmember.("MONTHNUM") but it is giving syntax error like invalid "/" in the formula and so on.

What is the correct syntax for dimension formula usage

Tried  below (BPC NW 7.5 SP11)

[ACCOUNT].[INV]/ ([TIME].CurrentMember.Properties("MONTHNUM"))   --  didn't work

[ACCOUNT].[INV]/strtovalue([TIME].currentmember.properties("MONTHNUM"))  -- didn't work

Any help would be appreciated.

-S