TMUA-P2-ERR1-004 — Paper P2 · Identifying errors in proofs · Level 2

Options

1. A
   The error is in line (1): $A$ and $B$ must be obtuse.
2. B
   The proof is fully valid for all angles.
3. C
   Line (3) is false for acute angles.
4. D
   Line (2) is unjustified as a general step: $\sin$ cannot be “cancelled” like a factor; only the conclusion happens to hold for acute angles since $\sin$ is strictly increasing on $[0^\circ, 90^\circ]$.
5. E
   Line (1) is impossible: two different acute angles cannot share a sine.

Letters only — the answer key is hidden. Work it out, then check the solution below.

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