G5
From Graphs to Algebra
The easiest way to find the equation of a straight line is to use the y-intercept and the slope. In the slope-intercept form y = ax + b, the y-intercept is the constant b. Next, determine the slope. If the y-intercept is (0,y') and the x-intercept is (x',0), then the slope b equals (y'/x'). Check that the sign of the slope is + if the line goes upward to the right, and - if the line goes downward to the right.
In the special case where the line is horizontal at the value y = b, the equation is just that (with no x). In the special case where the line is vertical at the value x = d, the equation is just that (with no y).
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