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# Line, Equations of

Resources

In mathematics, a line is a straight one-dimensional structure that has no thickness and extends in both directions, usually without end. An equation of a line is a mathematical statement about some aspect of a line. There are many different ways of writing the equation of a line in a coordinate plane. They all stem from the form ax + by + c = 0. Thus 2x + 3y - 5 = 0 is an equation of a line, with a = 2, b = 3, and c = -5. When the equation is written in the form y = mx + b one has the slope-intercept form: m is the slope of the line and b is the y-intercept. The equation 2x + 3y - 5= 0 becomes

So the line has slope -2/3 and a y-intercept 5/3.

When the equation is written in the form

one has the intercept form: a is the x-intercept and b is the y-intercept. The equation 2x + 3y -5 = 0 becomes

with x-intercept 5/2 and y-intercept 5/3.

When the equation is written in the form

where (x1, y1) and (x2,y2) are points on the line, one has the two point form. If one chooses the two points (1, 1) and (-2, 3) that lie on the line 2x + 3y-5 = 0, one have

When the equation is written in the form y-y1 = m (x-x1) where (x1, y1) is a point on the line, one has the point-slope form. If one chooses (-2, 3) as the point that lies on the line 2x + 3y = 0, one has y - 3 = -2/3 (x + 2).

In three space, a line is defined as the intersection of two non-parallel planes, such as 2x + y + 4z = 0 and x +3y+ 2z = 0. Standard equations of a line in three space are the two-point form:

where (x1,y1,z1) and (x2,y2,z2) are points on the line; and the parameter form: x = x1 + lt, y = y1 + mt, z = z1 + nt where the parameter t is the directed distance from a fixed point (x1,y1,z1) on the plane to any other point (x, y, z) of the plane, and l, m, and n are any constants.

## Resources

### BOOKS

Bittinger, Marvin L, and Davic Ellenbogen. Intermediate Algebra: Concepts and Applications. 6th ed. Reading, MA: Addison-Wesley Publishing, 2001.

Burton, David M. The History of Mathematics: An Introduction. New York: McGraw-Hill, 2007.

Lial, Margaret L. Precalculus. Boston, MA: Addison-Wesley, 2001.