10. Construct a right-angled triangle, being given the hypotenuse and the sum or difference of the sides.
11. The angles made with the base of an isosceles triangle by perpendiculars from its extremities on the equal sides are each equal to half the vertical angle.
12. The angle included between the internal bisector of one base angle of a triangle and the external bisector of the other base angle is equal to half the vertical angle.
13. In the construction of Prop. xviii. prove that the angle DBC is equal to half the difference of the base angles.
14. If A, B, C denote the angles of a △, prove that
(A + B),
(B + C),