Homepage › Solution manuals › Gilbert Strang › Introduction to Linear Algebra › Exercise 1.2.21
2v⋅w ≤ 2∥v∥∥w∥ leads to ∥v + w∥2 = v⋅v + 2v⋅w + w⋅w ≤∥v∥2 + 2∥v∥∥w∥+ ∥w∥2. This is (∥v∥ + ∥w∥)2. Taking square roots gives ∥v + w∥≤∥v∥ + ∥w∥