Mr Lyons attended Mathsconf5 with Luke Graham (Host of #mathschat) who put a call out to twitter collaborators to take on the hard to access maths topics.

@mrlyonsmaths was the first to reply and tackled G21 in this brilliant blog Remembering Trigonometric Values.

G21 - Better Maths blog - Collaboration in Maths

G21 can be found on page 31 of GCSE Mathematics (8300) for exams in May/June 2017 onwards.