I missed a lecture last week and am now struggling with my homework. This is the first question (and the easiest lol):

1. Denote by D the set of all triangles in the plane.

Define for ABC; A'B'C' in D the relation ABC ~_{s}A'B'C' if and only if ABC is similar to A'B'C'.

Prove that ~_{s}is an equivalence relation on D.

I've got my head around the basics of equivalence relations, I think but firstly I dont understand the notation my lecturer has used (~_{s }and ~_{c}~_{2}in other questions). If someone could explain the notation and give me a little help with this question I would be very grateful, and then I could do the rest of them myself

Thank you