Increasing / decreasing functions

1

12345678

Junior Member
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Mar 30, 2013
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Hello, I have a generic question about Increasing / decreasing functions.
I came across this example whilst doing a past paper
‘State a set of values for x for which 2x³ + 5x² - 4x is adecreasing function’.
My teacher said it was something to do with turning pointsbut I wonder if somebody could enlighten me on how to know this- the questionis only 2 marks so I assume it should be a quick method.
Do I have to plot a graph or is there a faster method?
Cheers- I know I haven’t shown any workings but I’m notreally sure what to show in this case.

 
12345678 said:
Hello, I have a generic question about Increasing / decreasing functions.
I came across this example whilst doing a past paper
‘State a set of values for x for which 2x³ + 5x² - 4x is adecreasing function’.
My teacher said it was something to do with turning pointsbut I wonder if somebody could enlighten me on how to know this- the questionis only 2 marks so I assume it should be a quick method.
Do I have to plot a graph or is there a faster method?
Cheers- I know I haven’t shown any workings but I’m notreally sure what to show in this case.
Click to expand...

The derivative of the function - slope of the tangent line - will be negative when the function is decreasing

The derivative of the function - slope of the tangent line - will be positive when the function is increasing.

At the turning point of the function - the derivative = 0.
 
Subhotosh Khan said:
The derivative of the function - slope of the tangent line - will be negative when the function is decreasing

The derivative of the function - slope of the tangent line - will be positive when the function is increasing.

At the turning point of the function - the derivative = 0.
Click to expand...

We were asked to work out the stationary points in a previous part. Do I just sub an x value either side into dy/dx to see if its positive or negative then?
 
12345678 said:
‘State a set of values for x for which 2x³ + 5x² - 4x is a decreasing function’.
My teacher said it was something to do with turning points...
Click to expand...
You posted this question to "Beginning Algebra", which suggests that you're about a year from taking calculus. Are you needing to figure out how to do this with only algebra? If so, are you allowed to use "technology"? ;)
 
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