Centre angle
Use the angle at the centre as twice the angle at the circumference on the same arc.
AQA 8300 · G10 subset · Higher
Circle theorems: find the mark that disappeared.
This reviewed journey isolates a centre-angle and tangent chain into three independently checked states, then asks for the method again after a delay and under exam conditions.
Reviewed family · Studara-authored items · No grade-uplift claim
The lost mark
One answer. Several places the method can fail.
Studara does not reduce the response to right or wrong. The reviewed contract checks these states independently.
Tangent fact
Place the right angle where the tangent meets the radius—not at a nearby point.
Final angle
Close the triangle only after both method states are correct.
Why it goes wrong
The diagnosis stays uncertainty-safe.
A wrong response can support a specific measured diagnosis. A blind multiple-choice diagnostic records only a possible cause until later working provides stronger evidence.
01
The theorem runs backwards
The centre angle is halved instead of doubled.
02
The 90° is misplaced
The tangent–radius fact is remembered but attached to the wrong vertex.
03
The answer lands; the working does not
A correct final angle with inconsistent method is recorded separately from a fully supported answer.
The worked repair
Repair the smallest broken step.
Teaching is not evidence. This sequence is the repair; the proof checks begin after it disappears.
- 1
Name the circumference angle and identify the arc it subtends.
- 2
Double it to establish the angle at the centre on that same arc.
- 3
Mark the tangent–radius angle as 90° at the point of contact.
- 4
Use the triangle angle sum to obtain the remaining angle.
What happens after the repair
Correction is the start, not the result.
Immediate
A different authored question
The repair disappears; a new value checks the chain without assistance.
Delayed
A parallel form later
A different item checks whether the same method can be retrieved after a real calendar delay.
Transfer
Values change
The invariant concept remains the G10 centre-angle and tangent chain. This is bounded surface transfer, not far transfer.
Exam
Three minutes, exam-like
The final form is timed, carries a mark tariff and requires an exam-like response.
REVIEWED BOUNDARY
This page covers one reviewed G10 centre-angle-to-tangent chain. Same-segment, semicircle, cyclic-quadrilateral, chord-bisector, alternate-segment and multi-theorem proof families are outside this reviewed journey.
Straight answers
Before you use this guide.
Which circle theorems does this journey cover?
The reviewed family joins the angle-at-centre theorem to the tangent–radius right angle and then the triangle angle sum. It does not claim coverage of every G10 theorem.
Does a correct final answer get all three states?
Only when the required centre-angle and tangent working is consistent with it. The deterministic contract records an answer that arrives through inconsistent working separately.
Is this an official AQA question?
No. Studara authors its own source-receipted items against the named specification point; it does not copy an examiner question.
Turn this method into evidence