Multiplying two variables

hello all,

Can I multiply two variables in CVX?
I mean that, in below line, "a" and "d" are variables, why can't I multiply them and what should I do? 
 x(1,1)*y(1)+x(1,2)*y(2)+aa*b(1)+a(3)*d(1,3)+a(2)*d(1,2)<=C_n(1)

FAQ: Why doesn’t CVX accept my problem? [READ THIS FIRST]