A number x in [-1, 1] has sin⁻¹x = 0.4 radians…

Step 1. The inverse cosine of $x$. $\cos^{-1}x = \dfrac{\pi}{2} - 0.4 = 1.5708 - 0.4 = 1.1708$.

Step 2. The inverse cosine of $-x$. $\cos^{-1}(-x) = \pi - \cos^{-1}x = 3.1416 - 1.1708 = 1.9708$.

Step 3. The inverse sine of $-x$. The inverse sine is odd, so $\sin^{-1}(-x) = -0.4$.