Assume ~S. Then from (2), you get ~C v ~D by Disjunctive Syllogism. Then you can write ~(C & D) from the previous statement using De Morgan. From this and (3) you get ~R by Modus Ponens. But, because R is given, that leads to R & ~R which is identically false. Since the assumption, ~S, led to a contradiction, ~S is false, therefore S.
John
My calculator said it, I believe it, that settles it
