Exercise 1748Linear algebra › Inner product spacesL1Added 3 Nov 2026
Orthogonal projection onto a subspace
#orthogonal projection#subspace#orthonormal basis
Statement
Let F=span((1,1,0),(0,1,1)) in R3 with the standard inner product. Compute the orthogonal projection of v=(1,2,3) onto F.
Hint
Find an orthonormal basis of F, then pF(v)=∑i⟨v,ui⟩ui.
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Solution
Orthonormal basis of F by Gram–Schmidt. Take a1=(1,1,0) and a2=(0,1,1). - u1=∥a1∥a1=2(1,1,0). - ⟨a2,u1⟩=21, so a2−⟨a2,u1⟩u1=(0,1,1)−21(1,1,0)=(−21,21,1), whose norm is 41+41+1=23. Hence u2=3/2(−1/2,1/2,1)=6(−1,1,2).