Trig equations are a mark-scheme minefield. A student can find a perfectly correct angle and still drop half the marks — because the marks aren't just for solving, they're for finding every solution in the range, in the right form. Here's how examiners award them.
For something like solve 2 sin x = 1 for 0 ≤ x ≤ 360°:
sin x = 0.5 → x = 30°).The second M and A marks are the ones students throw away.
Giving x = 30° but not 150° earns the first A1 and loses the second. The method mark for "second solution" is also lost if no valid method is shown. This is the single biggest trig mark-loss.
Working in degrees when the range is in radians (or vice versa) usually keeps the method marks but loses accuracy. For A-level, exact radian answers (π/6) are often required — a decimal can cost the mark.
Listing solutions beyond the stated interval can lose the final accuracy mark — the mark scheme wants the right set, not a superset.
On CIE especially, a correctly used identity (sin²+cos²=1) is often credited as "seen or implied" (soi) — so showing the substitution protects the method mark even if later arithmetic slips.
Award the first method and accuracy marks for the principal solution, then specifically check: did they find all solutions in the range (and only those), in the required form (exact/radians)? Point the student to the missing solution rather than just marking the answer incomplete — that's the habit that wins back the most marks.
Mark trigonometry — and the whole paper — point by point against your board's scheme.
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