Question

Why we write (sin 3π/5) as sin(π-2π/5)? What is the reason of writing it ?

Answer 1

unit circle

Explanation:

We write
`sin ((3pi)/5) = sin (pi - (3pi)/5)`
because, on the unit circle these 2 arcs (angles) `(3pi)/5` and
`(pi - (3pi)/5)` have the same sin value.