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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))F=\operatorname{span}((1,1,0),(0,1,1)) in R3\mathbb{R}^3 with the standard inner product. Compute the orthogonal projection of v=(1,2,3)v=(1,2,3) onto FF.

Hint

Find an orthonormal basis of FF, then pF(v)=∑i⟨v,ui⟩uip_F(v)=\sum_i\langle v,u_i\rangle u_i.

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Solution

Orthonormal basis of FF by Gram–Schmidt. Take a1=(1,1,0)a_1=(1,1,0) and a2=(0,1,1)a_2=(0,1,1).
- u1=a1∥a1∥=(1,1,0)2u_1=\dfrac{a_1}{\|a_1\|}=\dfrac{(1,1,0)}{\sqrt2}.
- ⟨a2,u1⟩=12\langle a_2,u_1\rangle=\dfrac{1}{\sqrt2}, so
a2−⟨a2,u1⟩u1=(0,1,1)−12(1,1,0)=(−12,12,1),a_2-\langle a_2,u_1\rangle u_1=(0,1,1)-\frac12(1,1,0)=\left(-\frac12,\frac12,1\right),
whose norm is 14+14+1=32\sqrt{\tfrac14+\tfrac14+1}=\sqrt{\tfrac32}. Hence
u2=(−1/2, 1/2, 1)3/2=(−1,1,2)6.u_2=\frac{\left(-1/2,\,1/2,\,1\right)}{\sqrt{3/2}}=\frac{(-1,1,2)}{\sqrt6}.

Projections.
⟨v,u1⟩=1+2+02=32,⟨v,u2⟩=−1+2+66=76.\langle v,u_1\rangle=\frac{1+2+0}{\sqrt2}=\frac{3}{\sqrt2},\qquad \langle v,u_2\rangle=\frac{-1+2+6}{\sqrt6}=\frac{7}{\sqrt6}.

Projection.
pF(v)=32 u1+76 u2=32(1,1,0)+76(−1,1,2).p_F(v)=\frac{3}{\sqrt2}\,u_1+\frac{7}{\sqrt6}\,u_2 =\frac32(1,1,0)+\frac76(-1,1,2).
Componentwise:
pF(v)=(32−76, 32+76, 146)=(13, 83, 73).p_F(v)=\left(\frac32-\frac76,\ \frac32+\frac76,\ \frac{14}{6}\right) =\left(\frac13,\ \frac83,\ \frac73\right).