The original worked lessons remain the main explanations. Use these focused reminders to check the conditions before you start.
Chords, minor segments and major segments
Use triangle geometry to find chords and central angles.
Subtract the correct triangle from a sector.
Trace the correct arc-plus-chord boundary; interpret tangents and overlap.
For the minor central angle \(0<\theta\le\pi\), chord \(\displaystyle c=2r\sin(\frac{\theta}{2})\) and minor segment area \(\tfrac12r^2(\theta-\sin\theta)\). Use RAD mode. A segment perimeter is arc plus chord, not arc plus two radii. The major segment is the rest of the disk.