R is the midpoint of segment PS. Q is the midpoint of segment RS. P is located at (8,10), and S is located at (12,-6). What are the coordinates of Q

Here is their argument. given the obtuse angle x, we make a quadrilateral abcd with ∠dab = x, and ∠abc = 90◦, and ad = bc. say the perpendicular bisector to dc meets the perpendicular bisector to ab at p. then pa = pb and pc = pd. so the triangles pad and pbc have equal sides and are congruent. thus ∠pad = ∠pbc. but pab is isosceles, hence ∠pab = ∠pba. subtracting, gives x = ∠pad−∠pab = ∠pbc −∠pba = 90◦. this is a preposterous conclusion – just where is the mistake in the "proof" and why does the argument break down there?

In the polynomial below, what number should replace the question mark to produce a difference of squares? x2 + ? x - 36

What is the gcf of 20c^3-5c^4+30c^6