Triangles
Pythagorean theorema² + b² = c²
30-60-90sides: 1 : √3 : 2
45-45-90sides: 1 : 1 : √2
areaA = ½ b h
interior angles sum180°
Trigonometry
SOH-CAH-TOAsin = opp/hyp
cos = adj/hyp
tan = opp/adj
Pythagorean identitysin²θ + cos²θ = 1
complementary anglessin θ = cos(90°−θ)
Circles & coordinate geometry
area / circumferenceA = πr² · C = 2πr
arc lengths = (θ/360) · 2πr
circle equation(x−h)²+(y−k)²=r²center (h,k), radius r
distance formulad = √((Δx)²+(Δy)²)
midpointM = ((x₁+x₂)/2, (y₁+y₂)/2)