# Give an example of a function, g(x), that has a local maximum at (−3, 3) and a local minimum at (3, −3).

giorgiana1976 | College Teacher | (Level 3) Valedictorian

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The function g(x) is of 3rd order or larger, since a quadratic function cannot have more than one extreme, either a minimum or a maximum point.

g(x) = ax^3 + bx^2 + cx + d

Since the function has a local maximum at (-3,3), then x = -3 is canceling the first derivative of the function.

We'll differentiate with respect to x.

g'(x) = 3ax^2 + 2bx + c

g'(-3) = 27a - 6b + c

But g'(-3)=0 => 27a - 6b + c = 0 (1)

Now, we'll calculate g(-3)=3.

g(-3) = -27a + 9b - 3c + d <=> -27a + 9b - 3c + d = 3 (2)

g'(3) = 0 <=> 27a + 6b + c = 0 (3)

g(3) = -3 <=> 27a + 9b + 3c + d = -3 (4)

We'll equate (1)=(3):

27a - 6b + c = 27a + 6b + c

We'll eliminate like terms:

-12b = 0 => b = 0

-27a + 9b - 3c + d + 27a + 9b + 3c + d = 3 - 3

We'll substitute b by 0 and we'll eliminate like terms:

2d = 0

d = 0

We'll substitute b and d by 0 in 27a + 9b + 3c + d = -3

27a + 3c = -3

We'll divide by 3:

9a + c = -1 (5)

We know that 27a + 6b + c = 0 and b = 0 =>

=> 27a = -c => 9a = -c/3 (6)

We'll replace 9a by (6):

-c/3 + c = -1

-c + 3c = -3 => 2c = -3 => c = -3/2

9a = 3/2*3 => 9a = 1/2 => a = 1/18

A possible function g(x), whose local maximum is at (-3 , 3) and local minimum is at (3 , -3), is the 3rd order function: g(x) = x^3/18 - 3x/2.