https://www.khanacademy.org/math/cc-fifth-grade-math/properties-of-shapes/imp-quadrilaterals-2/a/polygons-review
https://www.khanacademy.org/math/basic-geo/basic-geometry-shapes/basic-geo-quadrilaterals/v/quadrilateral-overview
https://www.khanacademy.org/math/basic-geo/basic-geometry-shapes/basic-geometry-shapes/basic-geo-classifying-triangles/v/scalene-isosceles-equilateral-acute-right-obtuse