①ANGLES
straight line $=180^\circ$ · vertically opposite equal · parallel: corresponding (F) & alternate (Z) equal
②TRIANGLES
angles $=180^\circ$ · exterior $=$ two far angles · isosceles: equal base angles · equilateral $=60^\circ$
③PYTHAGORAS
$H^2=O^2+A^2$ · shorter side: $O^2=H^2-A^2$
④CIRCLES
$D=2r$ · tangent $\perp$ radius · radius bisects a chord at $90^\circ$ · angle in semicircle $=90^\circ$ · two radii $=$ isosceles
⑤ENLARGEMENT
$|A'B'|=k|AB|$ · area $=k^2\times$ area
⑥SIMILAR
equal angles ⇒ $\dfrac{a}{p}=\dfrac{b}{q}=\dfrac{c}{r}$ · line $\parallel$ base splits sides in same ratio