Note:
color(red)((1)sec^-1x=cos^-1(1/x)(1)sec−1x=cos−1(1x)
color(red)((2)csc^-1x=sin^-1(1/x)(2)csc−1x=sin−1(1x)
color(red)((3)cos^-1(costheta))=color(red)(theta,where,theta in [0,pi](3)cos−1(cosθ)=θ,where,θ∈[0,π]
color(red)((4)sin^-1(sintheta))=color(red)(theta,where,theta in [-pi/2,pi/2](4)sin−1(sinθ)=θ,where,θ∈[−π2,π2]
Here,
sec^-1(sqrt2)=cos^-1(1/sqrt2)...toApply (1)
sec^-1(sqrt2)=cos^-1(cos(pi/4))
sec^-1(sqrt2)=pi/4in[0,pi].............toApply(3)
Now,
csc^-1(2)=sin^-1(1/2).....toApply(2)
csc^-1(2)=sin^-1(sin(pi/6))
csc^-1(2)=pi/6in[-pi/2,pi/2]..........toApply(4)