Question

How can know when to use SIN, Cosine, ans Tangent?

Answer 1

ACCORDING TO THE QUESTION. a """"trick"""" is there, whenever the question is given to prove RHS=LHS always change the term in such a way that it convert in sine. AND YOU WILL SEE THAT THE PROBLEM GETS SOLVE EASILY

Answer 2

"Opposite" is opposite to the angle θ
"Adjacent" is adjacent (next to) to the angle θ
"Hypotenuse" is the long one

Sine Function:
sin(θ) = Opposite / Hypotenuse
Cosine Function:
cos(θ) = Adjacent / Hypotenuse
Tangent Function:
tan(θ) = Opposite / Adjacent

therefore, Use them in reference to the specified angle.