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Contents
Equation of Straight Line
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Equation
of a Straight Line
The
general cartesian equation of a straight line is
y - y1
= m (x - x1)where
m is the gradient and (x1, y1) is a known point on the
line.
Alternatively,
the cartesian equation can be rewritten as
y = mx + cwhere
c is a constant.
 Example
1: Find the equation of the line that joins A (1, 4) and B (-3, 2).
Solution:Gradient
= (4-2)/(1+3) = 1/2
Substitute
gradient = 1/2, point A (1, 4) into general cartesian equation,
y - 4 = 1/2 (x - 1)
2y = x + 7
Example
2: Find the equation of the line that has gradient 2 and passes through (-2,
-5). If the line passes through (3, t), find the value of t.
Solution:Substitute
gradient = 2, point (-2, -5) into the general cartesian equation,
y + 5 = 2 (x + 2)
y = 2x + 1
At
point (3, t),
t = 2 (3) + 1 = 7
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