let A,B,C be three sets.if A belongs to B and B is the subset of C, IS IT true that A is the subset of C???? IF not , give an example .

No, it is not true.

Example: Let A = {1,2}, B = {{1,2},3,4} and C={{1,2},3,4,5}

Then A belongs to B and B is a subset of C,

But A is not subset of C as the elements of A are not in C.