1)how can we say that an element of a set can never be a subset of itself?

2)how can an equilateral triangle in a plane be a subset of a triangle in same plane?

subset is sets and sets are collection of elements.

we can say A is subset of B when

we can say A is proper subset of B when

In a plane there are lot of triangle now in those triangle there are some which equilateral so collection of equilateral triangles is a set which is smaller than or equal to set of triangles in the same plane.