Try this option (not for all the tasks!): 1) rule for n-angle is: S=180°(n-2), where n - number of the sides. S₁₅=180*13=2340°; 2) S₂₅=180*23=4140°; 4) this is deltoid; 5) this is paralelogram; 6) x=130°; y=50°; 10) x=90°; y=30° (it is deltoid); 12) not enough info for parallelogram (the same for deltoid); 14) it is enough: according to a property of a parallelogram (it means that a square and rombus are parallelograms); 15) it is enough according to the properties of parallelograms; 16) x=2; y=1; 18) poit S(0;0); point T(b+c;d). The coordinates of the midpoint are (0.5b+0.5c;0.5d); slope=arctg(d/(b+c)).
