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    • Thread Starter

    1) Triangle PQR has vertices P(-2, -1) Q(-7, 4) R(-1, 1)

    a) Show PR is perpendicular to QR.

    Now for this I thought if i found the gradients of both lines using y2-y1/x2-x1 then by showing they both multipy to make -1.

    But that gave me 8/5 x 3 which doesnt equal -1 so that doesnt show that they are perpendicular...so i think ive gone wrong here somewhere.

    2) PQ is a diameter of a circle. Find an equation of the circle when:

    a) P is (2, 0) and Q is (6,0)

    With this i assumed that if i found the midpoint of the line then that would give me the centre of the circle which I could then place into the equation :
    (x-a)^2 + (y-b)^2 = r^2

    Then to find the radius i thought by finding the magnitude of the line then i could find the diameter and half it.

    I got the answer to be (x-1)^2 + (y-3)^2 = 4 but the book tells me its
    (x-4)^2 + y^2 = 4

    Could someone attempt to explain where ive gone wrong on these two questions.


    1a) mPR=((dy)/(dx))=((1--1)/(-1--2))=(2/1)=2

    If two lines are perpendicular ((m1)(m2))=-1

    Let m1=mPR and m2=mQR. Therefore:

    ((m1)(m2))=((2)(-1/2))=-1 Hence PR is perpendicular to QR.

    Q. E. D.

    2a) P(2,0) Q(6,0) and PQ is the diameter of the circle. Find the mid-point to find the centre of the cirlce.


    => = (((8)/(2)),((0)/(2)))=(4,0)

    Cartesian equation of a circle:

    (((x-a)^2)+((y-b)^2))=(r^2) where a and b are the x and y co-ordinates, respectively, of the centre of the circle and r is the radius of the circle.


    r: The radius is the magnitude of the distance between one side of the diameter to the midpoint of the circle. Let us consider P(2,0). Since the co-ordinates of the centre are (4,0). We can tell that the radius is 4-2=2units.

    Therefore the equation of the respective circle is:


    =>(((x-4)^2)+(y^2))=(4) Q. E. D.

    • Thread Starter

    thanks alot.
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Updated: November 9, 2004

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