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Easy question

  • 20-04-2010 8:37pm
    #1
    Moromaster
    Registered Users, Registered Users 2 Posts: 237 ✭✭


    The teacher won't explain it again as I was absent, how do you do this question:

    L is the line 3x-y-11=0. The line K a(-3, 9) and b(6, r). K is perpendicular to L. Find R.


Comments

  • #2
    MathsManiac
    Registered Users, Registered Users 2 Posts: 1,595 ✭✭✭MathsManiac


    Moromaster wrote: »
    The teacher won't explain it again as I was absent, how do you do this question:

    L is the line 3x-y-11=0. The line K a(-3, 9) and b(6, r). K is perpendicular to L. Find R.

    I assume you meant to say that K passes through a and b.

    First rearrange the equation of L to put it in the form y=mx+c. Then the number in front of the x is the slope of L.

    To get the slope of K, use the formula for slope of a line given two points on it (in the tables). Your answer to this will have an r in it.

    If the two lines are perpendicular, the product of their slopes is -1.

    This gives you an equation with r in it that you can solve to find r.


  • #3
    Moromaster
    Registered Users, Registered Users 2 Posts: 237 ✭✭Moromaster


    What's C? We were given 'y-y1=m(x-x1)


  • #4
    Moromaster
    Registered Users, Registered Users 2 Posts: 237 ✭✭Moromaster


    Moromaster wrote: »
    What's C? We were given 'y-y1=m(x-x1)
    Would you mind just possibly showing an example of this kind of answer?


  • #5
    MathsManiac
    Registered Users, Registered Users 2 Posts: 1,595 ✭✭✭MathsManiac


    If you have the equation of a line and you rearrange it so that the y is on its own on the left, then you'll end up with:
    y = [something] x + [something]

    That's what is meant by being in the form y=mx+c.

    When the equation is in that form, then the number in front of x is the slope, m, and the number on its own is the value at which the line crosses the y-axis, (called the intercept).

    So, for example, if I want to find the slope of the line 2x+3y-5=0, I rearrange it as follows:

    Step 1: 3y = -2x + 5
    Step 2: y = (-2/3)x + (5/3)

    From this I know that the slope is (-2/3), (and that the line crosses the y-axis at (0, 5/3), which is not relevant here).

    Now, if you do the same in your question, you should find that the slope of L is 3.


  • #6
    2ygb4cmqetsjhx
    Closed Accounts Posts: 564 ✭✭✭2ygb4cmqetsjhx


    Deleted!


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  • #7
    Mellor
    Registered Users, Registered Users 2 Posts: 39,902 ✭✭✭✭Mellor


    I think your method is a little long winded, but not much.

    In this it's minor, but you don't need to change it to the form y=mx+c
    in ax+by+c form, the slope is -a/b

    to find the perp slope, invert and change the sign. 3 becomes -1/3

    then solve slope in one go, using y-y/x-x

    in the above it won't save a lot of time but in more complex ones, like JC or LC paper questions, it could be valuable,

    Ps sorry for the state of this post, iPod browsing


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