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Three graphs depicting an increasing function, a decreasing function, and a constant function.

Increasing and decreasing functions

The slope determines if the function is an increasing linear function    , a decreasing linear function    , or a constant function.

  • f ( x ) = m x + b  is an increasing function if  m > 0.
  • f ( x ) = m x + b  is an decreasing function if  m < 0.
  • f ( x ) = m x + b  is a constant function if  m = 0.

Deciding whether a function is increasing, decreasing, or constant

Some recent studies suggest that a teenager sends an average of 60 texts per day. http://www.cbsnews.com/8301-501465_162-57400228-501465/teens-are-sending-60-texts-a-day-study-says/ For each of the following scenarios, find the linear function that describes the relationship between the input value and the output value. Then, determine whether the graph of the function is increasing, decreasing, or constant.

  1. The total number of texts a teen sends is considered a function of time in days. The input is the number of days, and output is the total number of texts sent.
  2. A teen has a limit of 500 texts per month in his or her data plan. The input is the number of days, and output is the total number of texts remaining for the month.
  3. A teen has an unlimited number of texts in his or her data plan for a cost of $50 per month. The input is the number of days, and output is the total cost of texting each month.

Analyze each function.

  1. The function can be represented as f ( x ) = 60 x where x is the number of days. The slope, 60, is positive so the function is increasing. This makes sense because the total number of texts increases with each day.
  2. The function can be represented as f ( x ) = 500 60 x where x is the number of days. In this case, the slope is negative so the function is decreasing. This makes sense because the number of texts remaining decreases each day and this function represents the number of texts remaining in the data plan after x days.
  3. The cost function can be represented as f ( x ) = 50 because the number of days does not affect the total cost. The slope is 0 so the function is constant.
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Calculating and interpreting slope

In the examples we have seen so far, we have had the slope provided for us. However, we often need to calculate the slope    given input and output values. Given two values for the input, x 1 and x 2 , and two corresponding values for the output, y 1 and y 2 —which can be represented by a set of points, ( x 1 ,   y 1 ) and ( x 2 ,   y 2 ) —we can calculate the slope m , as follows

m = change in output (rise) change in input (run) = Δ y Δ x = y 2 y 1 x 2 x 1

where Δ y is the vertical displacement and Δ x is the horizontal displacement. Note in function notation two corresponding values for the output y 1 and y 2 for the function f , y 1 = f ( x 1 ) and y 2 = f ( x 2 ) , so we could equivalently write

m = f ( x 2 ) f ( x 1 ) x 2 x 1

[link] indicates how the slope of the line between the points, ( x 1, y 1 ) and ( x 2, y 2 ) , is calculated. Recall that the slope measures steepness. The greater the absolute value of the slope, the steeper the line is.

Graph depicting how to calculate the slope of a line
The slope of a function is calculated by the change in y divided by the change in x . It does not matter which coordinate is used as the ( x 2 ,   y 2 ) and which is the ( x 1 ,   y 1 ) , as long as each calculation is started with the elements from the same coordinate pair.

Questions & Answers

Ayele, K., 2003. Introductory Economics, 3rd ed., Addis Ababa.
Widad Reply
can you send the book attached ?
Ariel
?
Ariel
What is economics
Widad Reply
the study of how humans make choices under conditions of scarcity
AI-Robot
U(x,y) = (x×y)1/2 find mu of x for y
Desalegn Reply
U(x,y) = (x×y)1/2 find mu of x for y
Desalegn
what is ecnomics
Jan Reply
this is the study of how the society manages it's scarce resources
Belonwu
what is macroeconomic
John Reply
macroeconomic is the branch of economics which studies actions, scale, activities and behaviour of the aggregate economy as a whole.
husaini
etc
husaini
difference between firm and industry
husaini Reply
what's the difference between a firm and an industry
Abdul
firm is the unit which transform inputs to output where as industry contain combination of firms with similar production 😅😅
Abdulraufu
Suppose the demand function that a firm faces shifted from Qd  120 3P to Qd  90  3P and the supply function has shifted from QS  20  2P to QS 10  2P . a) Find the effect of this change on price and quantity. b) Which of the changes in demand and supply is higher?
Toofiq Reply
explain standard reason why economic is a science
innocent Reply
factors influencing supply
Petrus Reply
what is economic.
Milan Reply
scares means__________________ends resources. unlimited
Jan
economics is a science that studies human behaviour as a relationship b/w ends and scares means which have alternative uses
Jan
calculate the profit maximizing for demand and supply
Zarshad Reply
Why qualify 28 supplies
Milan
what are explicit costs
Nomsa Reply
out-of-pocket costs for a firm, for example, payments for wages and salaries, rent, or materials
AI-Robot
concepts of supply in microeconomics
David Reply
economic overview notes
Amahle Reply
identify a demand and a supply curve
Salome Reply
i don't know
Parul
there's a difference
Aryan
Demand curve shows that how supply and others conditions affect on demand of a particular thing and what percent demand increase whith increase of supply of goods
Israr
Hi Sir please how do u calculate Cross elastic demand and income elastic demand?
Abari
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Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
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