Minimizing Distance Between Two Lines

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To minimize the distance between two lines L1 and L2, the goal is to find the closest points x1a1 on L1 and b + x2a2 on L2. The distance to minimize is expressed as ||x1a1 - (b + x2a2)||. The approach involves setting the derivative of the squared distance function to zero to find critical points. The discussion also touches on the use of projections to simplify the calculations, but the original poster ultimately realized a misunderstanding of the problem. The correct method was clarified through the discussion.
bodensee9
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Can someone help with the folowing?
Suppose L1 is the line through the origin in the direction of a1 and L2 is the line through b in the direction of a2. I am supposed to find the closest points x1a1 and b+x2a2 on the two lines.

So I am trying to find the equations that would minize ||x1a1-x2a2-b||.

Not really sure what equations to write. I know that I'm trying to find some vector c so that (c-x1a1+x2a2+b)^2 will be the minimum. This means that if I take the derivative of the above, then the derivative will be zero. So, if I break c down into its components, would I get

2(c1-x1a1)+2(c2+x2a2+b)=0? Or, would I be trying to find the projection of x2a2+b onto a1? And if I do that, would be projection from a2 onto a1 be a1(a1Ta1)-1aT? But what about for a2? Not sure if that's right either. Thanks.
 
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Figured out. misunderstood the problem at first. thanks.
 
Question: A clock's minute hand has length 4 and its hour hand has length 3. What is the distance between the tips at the moment when it is increasing most rapidly?(Putnam Exam Question) Answer: Making assumption that both the hands moves at constant angular velocities, the answer is ## \sqrt{7} .## But don't you think this assumption is somewhat doubtful and wrong?

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