what theroms do you need to know (hl) :) :)
theorem of pythagoras, circle theorem, parallelogram theorem, angles in a triangle add up to 180 and an exterior angle is equal to the 2 opposite interior angles
+1 to the above as regards theorem *proofs* that are examinable.
Check page 18 of the syllabus:
Theorems: [Formal proofs are not examinable at OL. Formal proofs of theorems 4, 6, 9, 14 and 19 are examinable at HL.]
But don't forget there are 19 geometry theorems in total, plus corollaries, that you need to understand and be able to apply.
See syllabus for the list.
Eamonn - TheMathsTutor.ie