Range of sin−1x−cos−1x is
−3π2,π2
−5π3,π3
−3π2,π
0,π
Let y=sin−1x−cos−1x=π2−2cos−1x
Since 0≤cos−1x≤π ⇒-2π≤-2cos−1x≤0 ⇒π2-2π≤π2-2cos−1x≤0 ⇒−3π2≤y≤π2