Not using math more advanced that the 10th Grade (thus excluding the dot product) proof that:

a)The diagonals of a rhombus (a four-sided polygon in which every side has the same length) are perpendicular;

b)That connecting the medium points of consecutive sides of a quadrilateral will yield a parallelogram;

c)That if (x1;y1);(x2;y2); (x3;y3) and (x4;y4) are part of the same line then: (y2-y1)/(x2-x1)=(y3-y4)/(x3-x4).

Please help me.