求平面内两条直线的交点
2014-08-21 16:46
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The
![](http://upload.wikimedia.org/math/9/d/d/9dd4e461268c8034f5c8564e155c67a6.png)
and
![](http://upload.wikimedia.org/math/4/1/5/415290769594460e2e485922904f345d.png)
coordinates of the point of intersection of two non-vertical lines can easily be found using the following substitutions and rearrangements.
Suppose that two lines have the equations
![](http://upload.wikimedia.org/math/a/6/7/a67127a74c6f744f9ea34cd51206472f.png)
and
![](http://upload.wikimedia.org/math/7/5/4/754192eae6dac1f01f8c7b3741196d24.png)
where
![](http://upload.wikimedia.org/math/0/c/c/0cc175b9c0f1b6a831c399e269772661.png)
and
![](http://upload.wikimedia.org/math/9/2/e/92eb5ffee6ae2fec3ad71c777531578f.png)
are the slopes (gradients) of the lines and where
![](http://upload.wikimedia.org/math/4/a/8/4a8a08f09d37b73795649038408b5f33.png)
and
![](http://upload.wikimedia.org/math/8/2/7/8277e0910d750195b448797616e091ad.png)
are the y-intercepts of the lines. At the point where the two lines intersect (if they do), both
![](http://upload.wikimedia.org/math/4/1/5/415290769594460e2e485922904f345d.png)
coordinates will be the same, hence the following equality:
![](http://upload.wikimedia.org/math/6/d/3/6d36d8524473bc5b2307a658c0f74e3e.png)
.
We can rearrange this expression in order to extract the value of
![](http://upload.wikimedia.org/math/9/d/d/9dd4e461268c8034f5c8564e155c67a6.png)
,
![](http://upload.wikimedia.org/math/6/7/0/67017143e31bfd1a27023aedd3a3d980.png)
,
and so,
![](http://upload.wikimedia.org/math/b/a/0/ba0a21862aa7bb3a27bab64e38756264.png)
.
To find the y coordinate, all we need to do is substitute the value of x into either one of the two line equations, for example, into the first:
![](http://upload.wikimedia.org/math/7/5/3/753ca30408a6bf0a3ec46fb08698ec46.png)
.
Hence, the point of intersection is
![](http://upload.wikimedia.org/math/f/3/f/f3f00114cd81250f4f90308b6f96db64.png)
.
Note if a = b then the two lines are parallel. If c ≠ d as well, the lines are different and there is no intersection, otherwise the two lines are identical.
![](http://upload.wikimedia.org/math/9/d/d/9dd4e461268c8034f5c8564e155c67a6.png)
and
![](http://upload.wikimedia.org/math/4/1/5/415290769594460e2e485922904f345d.png)
coordinates of the point of intersection of two non-vertical lines can easily be found using the following substitutions and rearrangements.
Suppose that two lines have the equations
![](http://upload.wikimedia.org/math/a/6/7/a67127a74c6f744f9ea34cd51206472f.png)
and
![](http://upload.wikimedia.org/math/7/5/4/754192eae6dac1f01f8c7b3741196d24.png)
where
![](http://upload.wikimedia.org/math/0/c/c/0cc175b9c0f1b6a831c399e269772661.png)
and
![](http://upload.wikimedia.org/math/9/2/e/92eb5ffee6ae2fec3ad71c777531578f.png)
are the slopes (gradients) of the lines and where
![](http://upload.wikimedia.org/math/4/a/8/4a8a08f09d37b73795649038408b5f33.png)
and
![](http://upload.wikimedia.org/math/8/2/7/8277e0910d750195b448797616e091ad.png)
are the y-intercepts of the lines. At the point where the two lines intersect (if they do), both
![](http://upload.wikimedia.org/math/4/1/5/415290769594460e2e485922904f345d.png)
coordinates will be the same, hence the following equality:
![](http://upload.wikimedia.org/math/6/d/3/6d36d8524473bc5b2307a658c0f74e3e.png)
.
We can rearrange this expression in order to extract the value of
![](http://upload.wikimedia.org/math/9/d/d/9dd4e461268c8034f5c8564e155c67a6.png)
,
![](http://upload.wikimedia.org/math/6/7/0/67017143e31bfd1a27023aedd3a3d980.png)
,
and so,
![](http://upload.wikimedia.org/math/b/a/0/ba0a21862aa7bb3a27bab64e38756264.png)
.
To find the y coordinate, all we need to do is substitute the value of x into either one of the two line equations, for example, into the first:
![](http://upload.wikimedia.org/math/7/5/3/753ca30408a6bf0a3ec46fb08698ec46.png)
.
Hence, the point of intersection is
![](http://upload.wikimedia.org/math/f/3/f/f3f00114cd81250f4f90308b6f96db64.png)
.
Note if a = b then the two lines are parallel. If c ≠ d as well, the lines are different and there is no intersection, otherwise the two lines are identical.
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