 f (x) = mx + c
The linear function  f (x) = mx + c
The expression,  f (x) = mx + c or  y = a1x + a0,  we call linear function or a first degree polynomial function of one variable. Where, the constant m is called the slope or gradient and the constant c is the y-intercept.
The slope of a line is the ratio of its vertical change to its horizontal change, or it is the tangent of the angle between the direction of the line and the x-axis.
The y-intercept is the point of intersection between the graph of the function and the y-axis.
The zero of a function or the x-intercept is the value of the independent variable x at which the value of the function is zero.
The graph of the linear function  this expression, in the Cartesian coordinate plane, represents the translation of the source linear function
y = mx in the direction of the y-axis by y0 = c or the translation in the direction of the x-axis by x0 = - c/m
as is shown in the figure above.
Therefore, the linear expression we also write as
y = a1x + a0   or   y = a1(x - x0)   or   y - y0 = a1x,  where  y0 = a0
To find the zero or the x-intercept of the linear function set  y = 0  and solve the equation for x, i.e.,
y = 0   =>    0 = a1x + a0 Example:  Find the equation of the line that passes through the origin and the point A(-3, 2). Translate the line in the direction of the x-axis by x0 = - 3, then find its equation.
Calculate the slope m by plugging the coordinates of the point A into the equation  y =  mx,  By plugging x0 = - 3 into  y = a1(x - x0) Properties of the linear function
We examine the behavior of a function y = f (x) by moving from left to right in the direction of x-axis by inspecting its graph.
The linear function f (x) = mx + c, m > 0 is increasing, the graph rises from left to right, that is,
f (x1) <  f (x2)   for all  x1 < x2 If m < 0 linear function decreasing,   f (x1) >  f (x2)   for all  x1 < x2    i.e., the graph falls from left to right. Linear functions have a constant rate of increase or decrease.
A linear function changes the sign at the root or zero point.
 Thus, if m > 0, then f (x) < 0 for all while f (x) > 0 for all That is,  f (x) = mx + c, m > 0 is negative for all x less than the root, positive for all x greater than the root,
and at the root f (x) = 0.
 If m < 0, then f (x) > 0 for all while f (x) < 0 for all    College algebra contents C 