maths

• speed = distance / time
• density = mass / volume
• area of a triangle = base $\cdot$ height / two
• area of a parallelogram = base $\cdot$ height
• stratified sampling = $\frac{number\ in\ category\ }{total}\cdot\ sample\ size$
• directly proportional : $y\ \propto Kx$
• indirectly proportional : $y\ \propto\frac{k}{x^{2\ }}$
• sum of interior angle for regular polygon = $\left(n\ -2\ \right)\ \cdot\ 180$
• exterior angle of a regular polygon :

  $\frac{360}{n}$
• frequency density = frequency / class width
• capture recapture =

  $\frac{population}{sample\ 1\ }=$$\frac{sample\ 2\ }{marked\ }\$
• area of a sector :

  $\frac{angle}{360}\ \cdot\ \pi r^2$
• arc length

  $\frac{angle\ }{360\ }\cdot\pi d$
• gradient

  $\frac{y2_{ }-y1}{x2-\ x1}$