DOUBTS:URGENT!!

*)The relation "congruence modulo m" on the set I of all integers is
1. Equivalence relation
2. A partial order relation
3. Reflexive and symmetric
4. Reflexive and antisymmetric

*)What is AΔB in set language??

*)A pole of 50meter high stands on a building 250m high. To an observer at a height of 300m, the building and the pole subtend equal angles. The distance of the observer from the top of the pole is

1. 25√6

2. 25√3

3.50

4.25

*)tan nβ= n sinα cosα
----------------
1-n² sin²Î±

then tan(α-β)=??

*) If three vectors A, B, C forms adjacent sides of a tetrahedron making an angle α,β,γ with each other, then what is the angle made by one of sides with it's base(triangular part)??