This chapter is the heart of 3D problem-solving. The author uses a theorem-proof-corollary format to cement the relation:
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Between the theory and the exercise, there is a large set of worked-out examples. Do not skip these. Cover the solution with your hand, try to solve it, and then check his method. He often uses "tricks" (like solving plane problems via determinants) that are not explicitly stated in the theory. This chapter is the heart of 3D problem-solving