Homepage › Solution manuals › Gilbert Strang › Introduction to Linear Algebra › Exercise 1.2.23
cos β = w1∕∥w∥ and sin β = w2∕∥w∥. Then cos (β − a) = cos βcos α + sin βsin α = v1w1∕∥v∥∥w∥ + v2w2∕∥v∥∥w∥ = v⋅w∕∥v∥∥w∥. This is cos 𝜃 because β − α = 𝜃