# `f(x) = (x^2)/(x - 1)` Find the local maximum and minimum values of `f` using both the First and Second Derivative Tests. Which method do you prefer?

*print*Print*list*Cite

### 1 Answer

The **First Derivative f'(c)** can be interpreted as:

- derivative of f(x) when x=c

- slope of a tangent line at a point (c, f(c))

- instantaneous rate of change of f(x) at x=c. f

The** First Derivative test** is commonly used to determine the possible local extrema: local minimum point or local maximum point. These can also be called as relative maximum or relative minimum

Local extrema exists at x=c when f'(c) =0.

To predict the concavity of the function f(x) at x=c,we follow:

**f'(a) >0 and f'(b) <0** in the interval a<c<b implies concave down and **local maximum point** occurs at x=c.

**f'(a) <0 and f'(b) >0** in the interval a<c<b implies concave up and **local minimum point** occurs at x=c.

Sign table:

If f'(x) >0 or f'(x) = positive value then function f(x) is increasing or has a positive slope of tangent line (slant line going up).

If f'(x)< 0 or f'(x) = negative value then function f(x) is decreasing or has a negative slope of tangent line (slant line going down).

As for **Second Derivative Test**, a critical point at x=c such that f'(c) =0 and f"(c) is continuous around the region of x=c follows:

**f"(c) >0 then local minimum** occurs at x=c.

**f"(c) < 0 then local maximum** occurs at x=c.

f"(c) =0 the inflection point, local extrema or neither will occur at x=c.

A real inflection points occurs at x=c such that f"(c)=0 if concavity changes before and after x=c.

For the given function `f(x) = x^2/(x-1)` , we can solve for first derivative f'(x) using product rule or quotient rule.

Using Product Rule:

f(x) =x^2/(x-1) is the same as f(x) =x^2*(x-1)^(-1)

`f'(x) = 2x*(x-1)^(-1) + x^2 (-1)*(x-1)^(-2)`

`f'(x)= (2x)/(x-1) - x^2/(x-1)^2 `

`f'(x) = (2x*(x-1) -x^2)/(x-1)^2`

`f'(x)= (2x^2-2x-x^2)/(x-1)^2 `

`f'(x)= (x^2-2x)/(x-1)^2`

Applying first derivative test, let f'(x) =0

`(x^2-2x)/(x-1)^2` =0

Cross-multiply (x-1)^2 to the other side.

`x^2-2x =0`

Factoring common factor "x":

x(x-2)=0

Apply zero-factor property: a*b =0 if a=0 or b=0

x=0 and x-2=0 or x=2.

Sign table:

x -1 0 0.5 2 3

f'(x) `3/4` ---- -3 --- `3/4`

f inc. dec. inc.

Concavity <--down `nnn` --> <---up `uuu` --->

Based on the table f'(-1)>0 and f'(0.5) <0 indicates a **local maximum at x=0 **while f'(0.5) <0 and f'(3)>0 indicates a** local minimum at x=2.**

Solve for the second derivative f"(x) using product rule derivative on`f'(x)= (x^2-2x)/(x-1)^2 ` or `f'(x)= (x^2-2x)*(x-1)^(-2)` :

f"(x) `= (2x-2)(x-1)^(-2)+ (x^2-2x)*(-2)(x-1)^(-3)`

`= ((2x-2)(x-1)+ (-2x^2+4x))/(x-1)^(3)`

=` ((2x^2-4x+2)+ (-2x^2+4x))/(x-1)^(3)`

`= 2/(x-1)^(3)`

Applying second derivative:

f"(0) = 2/(0-1)^(3)

= 2/(-1)^3

=2/(-1)

=-2 negative value or f"(0)<0

then **f"(0)<0 indicates a local maximum at x=0**

f"(2) = 2/(2-1)^(3)

= 2/(1)^3

=2/(1)

= 2 positive value or f"(2)>0

then** f"(2)>0 indicates local minimum at x=2**

In my opinion, I would prefer the second derivative test in this problem since f"(x) can be easily simplified and there is no need to use additional x-values to be plug-in.

**Sources:**