# Thread: Uniqueness of a line

1. ## Uniqueness of a line

The question is:

(a,b) and (c,d) are two distinct points in R^2. Prove there exists a unique line passing through them.

I proved that a line exists:
y=[(d-b)/(c-a)]x + [b-(d-b)a/(c-a)]

But how do I prove that it is unique? I know the technique, you say that there are two and prove that they are the same, but how do I go about it in this problem? Thank you for your help!

2. Originally Posted by sfitz
The question is:

(a,b) and (c,d) are two distinct points in R^2. Prove there exists a unique line passing through them.

I proved that a line exists:
y=[(d-b)/(c-a)]x + [b-(d-b)a/(c-a)]

But how do I prove that it is unique? I know the technique, you say that there are two and prove that they are the same, but how do I go about it in this problem? Thank you for your help!
Assume that there are at least 2 lines:

$y = m_1 x + c_1$ and $y = m_2 x + c_2$.

Sub the given points into each:

$y = m_1 x + c_1$:

$b = a m_1 + c_1$ .... (1)
$d = c m_1 + c_1$ .... (2)

Solve simultaneously for $m_1$ and $c_1$.

$y = m_2 x + c_2$:

$b = a m_2 + c_2$ .... (3)
$d = c m_2 + c_2$ .... (4)

Solve simultaneously for $m_2$ and $c_2$.

Therefore establish that $m_1 = m_2$ and $c_1 = c_2$.

3. Originally Posted by mr fantastic
Assume that there are at least 2 lines:

$y = m_1 x + c_1$ and $y = m_2 x + c_2$.
.
This does not prove it. What happens if the lines are vertical? Then we cannot write y=mx+b. Instead we should use ax+by = c. This handles all possible lines.