9. The angles at E, G, and F are right angles. Lines CE and BF are parallel, so the alternate interior angles at B and C where the transversal AD crosses them are equal. The triangles of interest are similar by AA~.
10. See problem 7. The ratio is the same.
11. Hard to tell what rule they want you to state. Every axis-aligned right triangle with its hypotenuse on AD will have the same ratio of side lengths. (That's one of the features of the graph of a line. It has the same slope everywhere, measured over any distance.)
