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I came across this problem in a maths exam. I solved this by taking that a light ray passes in such a way that it takes least path. But as this was a maths exam, i was wondering if it can be solved using maths?

Let $A=(0,1)$ and $B = (1,1)$ in the plane $\mathbb{R}^{2}$. Determine the length of the shortest path from A to B consisting of the line segments AP; PQ and QB, where P varies on the x-axis between the points $(0, 0)$ and $(1, 0)$ and Q varies on the line $y = 3$ between the points $(0, 3)$ and $(1, 3)$

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The reflection argument is mathematics, not Physics. One can set it up as a two variable minimization problem, then Lagrange multipliers, but that is a lot uglier. –  André Nicolas Jan 28 '13 at 18:04
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@Phani Raj I noticed that you've accepted only one answer out of sete questions you've asked. If you get a satisfying answer to a question, you should accept your favorite answer. –  Git Gud Jan 28 '13 at 20:52

2 Answers 2

Consider A' and B' obtain by reflection i.e. A'=(0,-1), B'=(1,5). Then you consider any path APQB and notice that its length is equal to A'PQB'. But such a length should not be larger than the distance between A' and B' and since the points P and Q that you found give equality, they are the minimum.

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You Can try using Cauchy Schwarz inequality.

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