12d is a very nice question i must say
in 3u it's not rly applying to anything so you're rly just proving some series sum or divisibility
for 3u induction is free marks
idk about 4u i haven't done that yet
if you're taking asian 5, you need to fill up 3 remaining units in yr11 (4u doesn't count for yr11) so u kinda have to pick 3 units that you're planning to drop (if you're deadset asian 5)