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← 2020 HSC Extension 1 paper Question 13(c)

Variation 1 · answer guide

Inverse trigonometric functions — Sibling identity with sin

Question

Suppose and for .

(i) Show that .

(ii) Using part (i), or otherwise, show that .

Graph of $g(x)=\sqrt{1-x^2}$ on $[-1,1]$.
Graph of on .

This practice question was inspired by Question 13(c) in the 2020 NSW HSC Mathematics Extension 1 examination, © NESA. It is an original TestMum variation that practises the same mathematical idea, not an official NESA question.

Step by step

  1. Set up the solution: use the restricted domain and range that make the inverse function single-valued.(i) Differentiate : .
  2. Simplify the previous line carefully, keeping exact values where possible.Differentiate : .
  3. Simplify the previous line carefully, keeping exact values where possible.Hence .
  4. Simplify the previous line carefully, keeping exact values where possible.(ii) Then on , so is constant on (by continuity at the endpoints).
  5. Finish the calculation, then check that the result meets the question’s conditions.Test : and , so the constant is .
  6. State the final answer clearly in the form the question requested.Therefore on .

Common mistake: changing so many features that the new example no longer tests the same mathematical idea.

Verification: Standard right-triangle identity ; equal derivatives plus one shared value force equality.

Any mark labels are a TestMum study aid, not an official NESA marking allocation.

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