18. Never True,because a trapezoid can only have one pair of parallel sides while a parallelogram can have two.
19. Sometimes,because a rectangle can also be a square.So if you have a normal rectangle and a rectangle that is a square.these are not similar,but if you have two normal rectangles than these are similar.
20.Always,because if two isosceles triangles have the same vertex angle then there will only be a certain amount of degrees left for the other two corners.A isosceles triangle is symmetrical so there we will have to divide the few remaining degrees by two and give one to each angle.
