Easy question

Moromaster

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.

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.

Moromaster

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

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?

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.

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Deleted!

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