5. ABC is a triangle. If D is the midpoint of AC, show that BA + BC = 2BD. 6. ABCD is a quadrilateral with AB equal and parallel to DC. Prove that AD is equal and parallel to BC. 7. ABCD is a square and P, Q are the midpoints of BC, CD respectively. If AP = a and AQ = b, find in terms of a and b, the directed line segments (i) AB, (ii) AD, (iii) BD, (iv) AC. 8. ABC is a triangle and P is any point in BC. If PQ is the resultant of AP, PB, PC, show that ABQC is a parallelogram, and Q is therefore a fixed point.