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Use the definitions of increasing and decreasing to argue that f(x)=x4 is decreasing on (−∞,0] and increasing on [0,∞). Then use derivatives to argue the same thing.

Short Answer

Expert verified

The statement has been proven.

Step by step solution

01

Step 1. Given information

We have been given a function f(x)=x4.

We have to use the definitions of increasing and decreasing to argue that this function is decreasing on (−∞,0]and increasing on [0,∞).

02

Step 2. Using the definition

For a and b in the interval (-∞,0]

Now if a<b≤0

Then,

role="math" localid="1648442267515" a4>b4

Thus the function is decreasing in the interval (-∞,0]

Also,

For a and b in the interval role="math" localid="1648442301380" [0,∞)

Now if 0<a<bthen,

a4<b4

Thus the function is increasing in the interval[0,∞)

03

Step 3. Using the derivative

The derivative of the function is given by :

f′(x)=4x3

The function f′(x)=4x3is always negative for x<0

The function f′(x)=4x3is always positive for x>0

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