A pentagram (pent- means 5) is based on 5-sided figures but a trapezoid is a type of quadrilateral (quad- means 4), based on 4. A pentagram and a trapezoid can both be broken down into triangles, but otherwise they don't have much in common. A pentagram is a regular figure and looks like a 5-pointed star, often inscribed in a circle, so it's symmetrical whereas a trapezoid has no symmetry. Both figures are two-dimensional.