method for solving?

josh123

Junior Member
Joined
Oct 5, 2005
Messages
52
A line has a slope of -4 and passes thru the point (3, 2). Then an equation for that line is

Slope is very tricky for me. I did this one like the book shows and I get the answer of y = 3x + 15 but then do it based off my notes and get y = 3x - 10 so I am doing this wrong I think. Can someone show me a good method for solving?
 

Gene

Senior Member
Joined
Oct 8, 2003
Messages
1,904
The quick way of checking is to put in x=3 to your equation and see if y=2. Since it goes thru that point, if you were correct it would.
There is one formula to use when you know the slope, m. Well it comes in two "flavors" but it is the same.
y=mx+c if you know what the y intercept is (that is the form your "answer" is in, but you have m=3 so it can't be correct. m=-4)
or
(y-a)=m(x-b) when you know that it goes thru P(b,a)
In both the slope is m.
Since they tell you the point and the slope (P(3,2) & -4) the second is the one you want.
Just solve
(y-2)=-4(x-3)
for y.
 

josh123

Junior Member
Joined
Oct 5, 2005
Messages
52
Gene,

Do I want it to equal 2 or the slope -4 because y = 2x - 10 is equal to -4 which is the slope.
 

Gene

Senior Member
Joined
Oct 8, 2003
Messages
1,904
You want it to equal 2 'cause you are finding y, not m. It goes thru the point (x,y) = (3,2). That says when x=3, y=2. When it is in the form y=mx+c the multiplier of x (m) is the slope. A line with any slope can go thru a point. You are interested in the one with a slope m = -4.
 
Top