a,b,c are all positive variables,can this constraint be expressed in CVX?

-rel_entr(a,a+b) >= c

CVX will treat > as being >=. So if you need strict inequality, you should change it to -rel_entr(a,a+b) >= c + small_positive_number, where small_positive_number is perhaps 1e-5 or 1e-6.

Thank you sir, I wanna ask you about another expression
can log(1+c1g/(c2g+c3)) be expressed in CVX?
where g is variable