Hi, the question is from Serge Lang - Basic mathematics, Page 171 exercise 6.
Thing to prove:
Let ΔPQM and ΔP'Q'M' be right triangles whose right angles are at Q and Q', respectively. Assume that the corresponding legs have the same length:
d(P,Q)=d(P',Q')
d(Q,M)=d(Q',M')
Then the right...