arcsinx值域[-π/2,π/2]
arccosx值域[0,π],
x>0时,arcsinx值域[0,π/2]
arccosx值域[0,π/2]
arcsinx=u
sinu=x=cos(π/2-u)
arccosx=π/2-u
arcsinx+arccosx=π/2
x