Hint:
We know that ,
color(red)(sin^-1(sintheta)=theta,sin−1(sinθ)=θ, where, color(red)(theta in [-pi/2,pi/2]θ∈[−π2,π2]
Even though,
color(blue)(sin^-1(sin((5pi)/6))!=((5pi)/6),sin−1(sin(5π6))≠(5π6), because, color(blue)((5pi)/6 !in [-pi/2,pi/2]5π6∉[−π2,π2]
So, we reduce theta=(5pi)/6.θ=5π6. But how ? By using Addition and Subtraction Formulas for sine and cosine .Now enjoy the answer.
****Answer:****
Here,
(5pi)/6=pi-pi/65π6=π−π6
(sin^-1(sin((5pi)/6))=sin^-1(sin(pi-pi/6))(sin−1(sin(5π6))=sin−1(sin(π−π6))
=sin^-1(sin(pi/6))....to, Apply color(red)([sin(pi-theta)=sintheta ]
=pi/6......to, where, pi/6in[-pi/2,pi/2]