# Critical Thinking

• Aug 20th 2007, 08:39 PM
wpcp15
Critical Thinking

A line passes through A(3,7) and B(-4,9). Find the value of a if C(a,1) is on the line.

I thought it had something to do with finding the slope . . .
• Aug 20th 2007, 08:44 PM
ThePerfectHacker
Quote:

Originally Posted by wpcp15

A line passes through A(3,7) and B(-4,9). Find the value of a if C(a,1) is on the line.

I thought it had something to do with finding the slope . . .

1)Find the equation of the line y=mx+b

Now substitute x=a and y=1.

You have an equation for "a" solve for it.
• Aug 20th 2007, 09:06 PM
wpcp15
So m=-2/7 and y-int=-8

1=-2/7a+8

a=24.5

Is this right?
• Aug 20th 2007, 11:27 PM
earboth
Quote:

Originally Posted by wpcp15

A line passes through A(3,7) and B(-4,9). Find the value of a if C(a,1) is on the line.

I thought it had something to do with finding the slope . . .

Hello,

if you have 2 separate points $P_1(x_1 , y_1)~\text{and}~P_2(x_2 , y_2)$ which lay on a line then the equation of the line is:

$\frac{y - y_1}{x - x_1} = \underbrace{\frac{y_2 - y_1}{x_2 -x_1}}_{\text{slope of the line}}$ . this formula is called two-points-formula of a straight line.

Plug in the coordinates of the points. Don't forget with your problem x = a and y = 1. You'll get:

$\frac{1 - 7}{a - 3} = \frac{9 - 7}{-4 - 3}$ . Solve for a:

$\frac{-6}{a-3} = \frac{2}{-7}~\Longrightarrow~42 = 2a - 6 ~\Longrightarrow~ 2a = 48 ~\Longrightarrow ~ \boxed{a = 24}$
• Aug 20th 2007, 11:43 PM
DivideBy0
Refreshing approach earboth, I usually go through the process of finding the full equation first.