Area: circle A=πr², rectangle A=ℓw, triangle A=½bh. Circumference C=2πr. Pythagorean theorem a²+b²=c².
Special right triangles: 30°-60°-90° sides x : x√3 : 2x; 45°-45°-90° sides s : s : s√2.
Volume: box V=ℓwh, cylinder V=πr²h, sphere V=(4/3)πr³, cone V=(1/3)πr²h, pyramid V=(1/3)ℓwh.
The sum of the angles in a triangle is 180°. A circle has 360° (2π radians); a straight angle is 180°.