# Concurrent Lines

Forums Leaving Cert Maths Concurrent Lines

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• #13111

Find the equation of the line through the point of intersecton of the lines x-2y+4=0 and 3x+4y-8=0 and which also contains the origin.

• #376

:Find the equation of the line through the point of intersecton of the lines x-2y+4=0 and 3x+4y-8=0 and which also contains the origin.

• #13112

Email me at @yahoo.co.uk">lisawilliamandy@yahoo.co.uk if you know the answer. Thanks

• #13113 Anonymous

On this page, we hope to clear up problems that you might have with finding the equation of a line.

Many times, you'll have the graph of an equation shown to you and you'll need to find the equation. This seems a very daunting task, but it's actually quite easy!

For example, take a horizontal line such as y = 2. Every point on that graph is 2 units above the x axis. All horizontal lines have equations that are written in the same format, such as y = -4.5. Because they're all written the same way, we can come up with a general formula for horizontal lines. It is the following equation (where k represents any real number): y = k. Under the same assumptions, the general formula for vertical lines can be written as follows (where k represents any real number): x = k.

Linear equations such as y = .009x + 34 also have a general equation that can represent any linear equation. It is written as follows: y = mx + b.

The things to remember about the above formula, which is called the slope-intercept formula, are outlined below.

1. Since you know a line with an equation in that form cannot be horizontal or vertical, all you need to find are m and b to find the equation.

2. b is called the intercept. It is the point when the line crosses the y axis.

3. m is called the slope. The slope has both a sign (either + or -) and a value (the number behind the sign). For example, the equation y = -3x + 4 has a negative sign and a value of three. Therefore, the slope is -3. When looking at a graph, you can always tell if the slope is negative or positive by the direction it points. When looking for the sign of a slope, look at the left side of the graph. Then, look at the right side of the graph. If the right side is lower than the left side, the line has a negative slope, if the right side is higher, the line has a positive slope.

Example: Example Graph

To find the value of the slope, you compute the rise over the run To do that, pick two points on the line at random and then draw a line through each of those points that run parallel to the coordinate axes. Count the number of units between the point on the line and where the two addition lines you drew intersect. The number of units on the horizontal line is the run and the number of units on the vertical line is the rise. Dividing the rise by the run gives you the value of the slope.

Example: Example Graph

Example: 1. Find the equation of the line graphed in the accompanying figure:

Accompanying Figure

Solution: The desired equation is in the

slope-intercept form. You need

to find m and b.

By looking at the graph, we can see

that b must be 3.

Also, by inspecting the graph, you

can see that the slope is negative

because the left end of the line is

higher than the right end.

Now you need to find the value of

the slope. Pick any two points on

the line and draw a line through

each point that is horizontal to

the coordinate axes. (For simplicity's

sake, we will use b, which is

(0,3) and the point given in the

problem, (2,0).) Count the

number of units on the vertical line

and horizontal line. The vertical

line is the rise and the

horizontal line is the run.

Compute the slope's value by putting

the rise over the run. In this

case, the rise is 3

and the run is 2.

Therefore, the value of the slope is

(3/2). Combine the sign of

the slope and the value of the slope

to get the complete number for m.

It is -(3/2).

Plug the values you found into the

slope-intercept general

equation and you get the following:

3

y = – -x + 3

2

Content from http://www.thinkquest.org

• #13114 Anonymous

While the last answer goes into much detail (but is good should you follow it)

Simply find the point of intersection between the 2 lines (by simultaneous equations (cancel to find x, sub the value you find in for x, and find y, (x,y) point found)

Find the slope btween the point you just found and the origin (0,0)

Then using the Equation of a line put in the point you found (or easier put in the oriigin (0,0) for y1 & x1 (because its in this question … warning only use this if the origin is in the q))

Woila out should pop the equation for your line