F(x)= 3x^4 + 3x^3 +8x +10  Find the interval where F(X) is concave down  round answer to 4 decimal places

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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F(x)= 3x^4 + 3x^3 +8x +10 

Find the interval where F(X) is concave down 

round answer to 4 decimal places 

Expert Solution
Step 1

The given function is fx=3x4+3x3+8x+10.

Evaluate the inflection points as follows.

f''x=0f'ddx3x4+3x3+8x+10=0ddx12x3+9x2+8=036x2+18x=018x2x+1=0x=0  or  x=-12

Thus, the inflection points of the given function are x=0 and x=-1/2.

The intervals to consider are  -,-12,-12,0  and  0,.

Step 2

A function is concave upward if f''x>0 and concave downward if f''x<0.

Consider a value x=-1 in the first interval -,-12.

Evaluate the sign of f''x as follows.

f''x=36x2+18xf''-1=36-12+18-1f''-1=36-18f''-1=18f''-1>0

So, the function is concave upward in the interval -,-12.

 

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