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Math Help - I can't pretend to subtend

  1. #1
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    Talking I can't pretend to subtend

    Hi guys,

    I'm having a bit of trouble understanding the word "subtends". Here's the question:

    Prove that the equation of the chord joining the points P(cp, c/p) and Q(cq, c/q) on the rectangular hyperbola xy = c^2 is pqy + x = c(p + q)

    This was OK, my problem is the next bit...

    It is given that PQ subtends a right angle at the point R(cr, c/r) on the curve. Prove that PQ is parallel to the normal at R to the curve.

    I can't quite understand what this first sentance means. How does it look?
    In my attachment the curve xy = c^2 is accurately drawn, but my choice of P, Q and R don't fit the requirement. I have tried moving the chord PQ around to get it parallel to a Normal at some other point R on the curve and I can't do it! It should be easy! Have I understood the meaning of the word subtend? I looked it up: "to be opposite to and delimit" but this doesn't help.

    Can anyone see how this should look on the diagram?
    Attached Thumbnails Attached Thumbnails I can't pretend to subtend-m16g19.jpg  
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  2. #2
    Super Member malaygoel's Avatar
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    Quote Originally Posted by s_ingram View Post

    It is given that PQ subtends a right angle at the point R(cr, c/r) on the curve. Prove that PQ is parallel to the normal at R to the curve.
    It means that anglePRQ is a right angle.
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  3. #3
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    Does it! OK. Now I'll try and figure out the proof. Thanks.
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