Home page for accesible maths Math 101 Chapter 2: Functions of a real variable

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2.33 The inverse sine function sin-1\sin^{-1} or arcsin

The function sin::\sin:{\mathbb{R}}\rightarrow{\mathbb{R}}: sinx\sin x does not have an inverse since the equation sinx=0\sin x=0 has many solutions.

Example (Inverse sine)

The function sin:[-π/2,π/2][-1,1]\sin:[-\pi/2,\pi/2]\rightarrow[-1,1] has an inverse function sin-1:[-1,1][-π/2,π/2]\sin^{-1}:[-1,1]\rightarrow[-\pi/2,\pi/2].

We cut down the codomain to the range [-1,1][-1,1], so that sin\sin takes all the values in the codomain. Then we cut down the domain to [-π/2,π/2][-\pi/2,\pi/2] so as to make the modified function strictly increasing. Then we can apply Proposition 2.32 to obtain the inverse function.