Question

what is the exaxt value of sin (3 pi/14)

Answer 1

It is already exactly value. Do you mean decimal approximation?

Answer 2

usually the angle given are negative and in quadrant 2,3 or 4, so we need some step in
finding its exact location
step 1) in finding the exact value is to locate the terminal side
of the angle,or the positive coterminal angle if its negative
step2)find the reference angle to the angle in question
if the angle is in the first quadrant no need to find it

this time (3 pi/14) is in the first quadrant so no need to find its coterminal or reference angle