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Exercise 1055Linear algebra › Matrices and linear systemsL1Added 3 Nov 2026

Gram-Schmidt orthogonalization

#Gram-Schmidt#orthogonalization#orthonormal basis

Statement

Apply Gram-Schmidt to the vectors v1=(1,1,0)v_1 = (1,1,0), v2=(1,0,1)v_2 = (1,0,1), v3=(0,1,1)v_3 = (0,1,1) in R3\mathbb{R}^3.

Hint

At each step, subtract from vkv_k its projection onto the previously constructed orthonormal vectors.

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Solution

Step 1. u1=v1/∥v1∥=(1,1,0)/2u_1 = v_1 / \|v_1\| = (1,1,0)/\sqrt2.

Step 2. Projection of v2v_2 onto u1u_1: ⟨v2,u1⟩=1/2\langle v_2, u_1 \rangle = 1/\sqrt2. So
w2=v2−(1/2)u1=(1,0,1)−(1/2)(1,1,0)=(1/2,−1/2,1).w_2 = v_2 - (1/\sqrt2)u_1 = (1,0,1) - (1/2)(1,1,0) = (1/2,-1/2,1).
Norm: 1/4+1/4+1=3/2\sqrt{1/4+1/4+1} = \sqrt{3/2}, so u2=(1/6,−1/6,2/3)u_2 = (1/\sqrt6, -1/\sqrt6, \sqrt{2/3}).

Step 3. Projections of v3v_3:
⟨v3,u1⟩=1/2,⟨v3,u2⟩=(−1+1+2)/6=2/6.\langle v_3, u_1 \rangle = 1/\sqrt2, \qquad \langle v_3, u_2 \rangle = (-1+1+2)/\sqrt6 = 2/\sqrt6.
So
w3=v3−(1/2)u1−(2/6)u2=(0,1,1)−(1/2)(1,1,0)−(1/3)(1,−1,2)=(1/6,5/6,1/3).w_3 = v_3 - (1/\sqrt2)u_1 - (2/\sqrt6)u_2 = (0,1,1) - (1/2)(1,1,0) - (1/3)(1,-1,2) = (1/6, 5/6, 1/3).
Norm: 1/36+25/36+4/36=30/6\sqrt{1/36+25/36+4/36} = \sqrt{30}/6, so u3=6/5(1/6,5/6,1/3)=(1/30,5/30,2/30)u_3 = \sqrt{6/5}(1/6, 5/6, 1/3) = (1/\sqrt{30}, 5/\sqrt{30}, 2/\sqrt{30}).

Conclusion. The orthonormal basis is
u1=12(1,1,0),u2=16(1,−1,2),u3=130(1,5,2).u_1 = \frac{1}{\sqrt2}(1,1,0), \quad u_2 = \frac{1}{\sqrt6}(1,-1,2), \quad u_3 = \frac{1}{\sqrt{30}}(1,5,2).