How do you write an equation from a graph? we are given a square grid with Y axis numbered with 4, 2, 0, -2, -4 and then X axis with -4, -2, 0, 2, 4. and then a line intersecting at y at the 2 and x at the 2. we haven't really tried anything because we don't know where to start. Thanks.
First watch this video:
http://www.youtube.com/watch?v=Mbd-wT0aMQ0&feature=related
After watching that, then continue:
This is what you have:
Start at the y-intercept and draw a line straight down any arbitrary
number of blocks, say 3 units straight down vertically, like this
green arrow below:
Now starting at that arrowhead, draw another green arrow over to the
right over to the line, like this:
1. We find the slope, which is a fraction,
A. The numerator (rise) of the slope fraction is equal to the length
of the first arrow, the vertical one going down. Its length is 3, but
since the arrow went DOWN instead of up, the numerator, of the
slope is NEGATIVE, so it's -3.
B. The denominator (run) of the slope fraction is equal to the length
of the second arrow, the horizontal one going right. Its length is 3,
and since the arrow went RIGHT instead of left, the denominator of the
slope is POSITIVE, so it's +3.
C. So the slope is the fraction which reduces to -1.
D. We use the letter "m" to represent ths slope so, m = -1
2. We observe the y-intercept,
1. This is the number on the y-axis where the line crosses the y-axis.
That number is 2.
2. We use the letter "b" to stand for the number on th y-axis where
the line crosses the y-axis.
3. We substitute the values for "m" and "b" in the slope-intecept
formula, which is
y = mx + b
We substitute the values m=-1 and b=2 into that equation:
y = -1x + 2
4. That is the equation of the line. We don't have to write the 1, and
just write
y = -x + 2
Edwin