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MAT091 :: Lecture Note :: Week 13
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Functions are sometimes categorized as either increasing or decreasing.
An increasing function is a function whose output values increase (i.e. get larger) when the inputs increase.
f(0) = 5 f(1) = 10 f(2) = 15 f(3) = 20 f(x) appears to be an increasing functionA decreasing function is a function whose output values decrease (i.e. get smaller) when the inputs increase.
g(0) = 100 g(1) = 90 g(2) = 80 g(3) = 70 g(x) appears to be a decreasing functionA constant function
f(x) = k
(where 'k' is a constant) is neither increasing nor decreasing.The identity function
f(x) = x
is an increasing function.Some functions can be both increasing and decreasing. For example, over an interval A, a function might be increasing, while it decreases over an interval B. Review the absolute value function.
abs(x) is decreasing from (INF, 0) abs(x) is zero at 0 abs(x) is increasing from (0, INF)Technical definitions for increasing and decreasing functions.
increasing function: ==================== Function f(x) increases on an interval I if f(b) > f(a) for all b > a, where a,b are in I. decreasing function: ==================== Function f(x) decreases on an interval I if f(b) < f(a) for all b > a, where a,b are in I.
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MW.com defines linear as follows.
"of, relating to, resembling, or having a graph that is a line and especially a straight line" [...and...] "having or being a response or output that is directly proportional to the input"Observe that the word linear contains the word line.
A linear function is a function is a "first degree polynomial function of one variable. These functions are called 'linear' because they are precisely the functions whose graph in the Cartesian coordinate plane is a straight line."
The following are equations for a line.
slopeintercept form: y = mx + b ... used when slope and yintercept are known pointslope form: y  y_{1} = m(x  x_{1}) ... where (x_{1}, y_{1}) is a point on the line having slope m standard form: Ax + By = CSlope is another way of saying "rate of change" and linear functions have a constant rate of change.
Slope is often described as the ratio "rise over run."
Linear functions with a positive constant rate of change are increasing functions. Linear functions with a negative constant rate of change are decreasing functions.
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The slope of a line is a measure of its average rate of change (steepness?). The slope also indicates if the line is increasing (uphill?) or decreasing (downhill?). In linear equations, slope is represented by the letter m.
The following is how to find the slope between two unique points on a line.
point 1: (x_{1}, y_{1}) point 2: (x_{2}, y_{2}) slope = m = (y_{2}  y_{1}) / (x_{2}  x_{1}) note: x_{1} ≠ x_{2}Slope represents the ratio of changeinoutput over changeininput.
m = Δy / Δx (change in output / change in input) Δ is the "delta" character Δy is read "change of" (Δy is read "change of" y) Δy = y_{2}  y_{1}Slope is often described as the ratio "rise over run."
rise m =  runSome slope notes.
horizonal lines have slope 0 (y = f(x) = k, where k is a consant) vertical lines have undefined slope (x = k, where k is a consant) parallel lines have equal slopes (m_{1} = m_{2}) perpendicular lines have negative inverse slopes (m_{2} = 1/m_{1})The following is a verbose form the slopeintercept equation.
output = slope * input + initial_value_if_any ...or... output = average_rate_of_change * input + initial_value_if_any
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We frequently need to know the points where lines (graphs in general) intersect (cross) the horizonalaxis and verticalaxis (often called the xintercept and yintercept, respectively).
The verticalintercept for a function is found by evaluating the function when its input is zero. The orderdpair for a verticalintercept will be
(0, something)
.The horizontalintercept for a function is found by setting its output to zero and finding what input value results in that output. The orderdpair for a horizonalintercept will be
(something, 0)
.f(x) = 5(x) + 3 verticalintercept (yintercept)... set the input to 0 and evaluate: f(0) = 5(0) + 3 = 3 verticalintercept is (0, 3) horizontalintercept (xintercept)... set the output to 0 and solve: 0 = 5(x) + 3 3 = 5(x) 3/5 = x horizontalintercept is (3/5, 0)Generalizations.
given f(x) = m(x) + b verticalintercept: (0, b) [notice input is 0] horizontalintercept: (b/m, 0) [notice output is 0]
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