Question 8**+Find the fourth vertex of the parallelogram with adjacent vertices \(0\), \(2+i\), and \(-1+4i\).
Question 12***+A student says \(|z_1+z_2|=|z_1|+|z_2|\) for all complex numbers. Give a counterexample and explain it geometrically.
Question 13****The points \(0\), \(z\), \(w\), and \(z+w\) form a parallelogram. For \(z=2-3i\) and \(w=-5+i\), find both diagonals as complex displacements.
Question 14****Find \(z\) if the parallelogram with vertices \(0\), \(2+i\), \(z\), and \(5-2i\) has \(5-2i\) as the vertex opposite \(0\).
Question 18*****Prove the parallelogram identity \(|z+w|^2+|z-w|^2=2|z|^2+2|w|^2\) for \(z=a+bi\), \(w=c+di\).
Question 19*****Show that equality in \(|z+w|\le |z|+|w|\) holds for \(z=2(1+i)\) and \(w=5(1+i)\), and explain why.
Question 20*****A learner claims that if \(|z+w|=|z-w|\), then \(z=0\) or \(w=0\). Disprove the claim geometrically with a complex example.