SOLUTION: I have a graph plotted (7,5) and (8,3) , horizontal axis is D and vertical axis is P. i need to get the equation of the line joining these points, i.e) P= 1d - 2 but it isnt that a

Algebra ->  Coordinate Systems and Linear Equations -> SOLUTION: I have a graph plotted (7,5) and (8,3) , horizontal axis is D and vertical axis is P. i need to get the equation of the line joining these points, i.e) P= 1d - 2 but it isnt that a      Log On


   

Question 49042: I have a graph plotted (7,5) and (8,3) , horizontal axis is D and vertical axis is P. i need to get the equation of the line joining these points, i.e) P= 1d - 2 but it isnt that and i cant work it out, any help would be appreciated.
Answer by rchill(405) About Me  (Show Source):
You can put this solution on YOUR website!
If you have two points, you can determine the equation of a line in the form of y=mx+b, where m is the slope. The slope, m, is defined as m=%28y%5B2%5D-y%5B1%5D%29%2F%28x%5B2%5D-x%5B1%5D%29. It doesn't matter which ordered pair is x%5B1%5D,y%5B1%5D and x%5B2%5D,y%5B2%5D because you'll get the same answer. Using substitution we get m=%285-3%29%2F%287-8%29 which reduces to m=-2. So our formula so far is y=-2x+b. To find out what b is, substitute one of your points (both x and y) into the equation; using the first ordered pair we get 5=-2*7+b, which simplifies to b=19. So the equation is y=-2x+19. Note that you'll get the exact same if you were to use the other ordered pair.

Because you need the equation with D and P instead of x and y, the formula becomes P=-2D%2B19.