Opgave 2.3: Udsnit 2 halvcirkler er indsat i et kvadrat med sidelængden 10
0) Hvad er orealet of det røde område?

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?

Aportion of a line bounded by two points is defined as

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?