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What characteristics of the graph of a function can we discuss by using the concept of...

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sw8-girl | Student, Undergraduate | (Level 1) Honors

Posted July 9, 2012 at 7:05 AM via web

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What characteristics of the graph of a function can we discuss by using the concept of differentiation (first and second derivatives).

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embizze | High School Teacher | (Level 1) Educator Emeritus

Posted July 9, 2012 at 4:05 PM (Answer #1)

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(1)Using the first derivative we can tell the intervals where a function is increasing or decreasing -- a positive 1st derivative indicates the function is increasing while a negative 1st derivative indicates the function is decreasing.

We can also find extrema (maxima, minima) -- extrema only occur when the 1st derivative is zero or fails to exist. Locating these points and applying the 1st derivative test tells us whether we have a local maximum or local minimum. By using our knowledge of the intervals where the function is increasing/decreasing, we can also say if the extrema are global (absolute maximum/minimum).

(2) Using the 2nd derivative we can discuss concavity -- how the function is curved. (This is the rate of increase/decrease.) If the function is increasing and concave up, it is increasing rapidly (like an exponential) but if it is increasing but concave down the rate of growth is slowing (like a square root function)

We can also find points of inflection; this is where the graph of a function changes concavity.

The second derivative test can be used to identify whether an extrema indicated by the first derivative is a maxima,minima, or point of inflection. (If the graph is concave down the point is a maximum, if concave up a minimum.)

 

Sources:

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najm1947 | Elementary School Teacher | (Level 1) Valedictorian

Posted July 9, 2012 at 9:53 AM (Answer #2)

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Let the function whose graph is drawn be y = f(x).

The first derivative of a fuction f'(x) gives us the slope of a function, and hence its graph. if it is equal to zero at a certain point then it tells that the slope of the function at that point has become parallel to the x-axis.

The second derivative f"(x) at that point tells us about the rate of change of the slope at that point.

If f"(x) is negative where f'(x) = 0, then it exhibits the maxima condition on the graph, i.e. graph makes a peak at that point

If f"(x) is positive where f'(x) = 0, then it exhibits the minima condition on the graph, i.e. graph makes a valley at that point

If f"(x) is also zero where f'(x) = 0, then it exhibits a point of contra-flexture, i.e. the graph gets parallel to the x-axis but resumes its upwards or downwards direction beyond this point.

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