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Find the acute angle from line directions

Find the acute angle between l1:r=(4,3,1)+λ(1,2,1)l_1:\mathbf r=(4,3,1)+\lambda(1,2,1) and l2:r=(−1,0,−1)+μ(3,2,6)l_2:\mathbf r=(-1,0,-1)+\mu(3,2,6), with λ,μ real.

CJC · Lecture · Example 26

Choose a point on the line and a nonzero direction; keep one parameter consistent across all components.

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