Let 𝑓(𝑥) = 1 3 𝑥 3 + 𝑥 2 − 15𝑥 − 9 . Use detailed sign tables in answering the following questions. (a) Find the intervals in which 𝑓 is increasing or decreasing. (b) Find the intervals in which the graph of 𝑦 = 𝑓(𝑥) is concave upwards or downwards.
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Expert's answer
2021-04-29T16:54:55-0400
As I understood, the function is f(x)=13x3+x2−15x−9 .
a) Then, intervals in which f(x) is increasing ↔f′(x)>0 .
Intervals in which f(x) is decreasing ↔f′(x)<0 .
f′(x)=39x2+2x−15⟶ find nulls of f′(x)
x1,2=781(−2±4+4∗15∗39)=391(−1±586),x1<x2
We have found null of quadratic trinomial, so since it's "branches" are directed up :
x<x1⟶f′(x)>0x1<x<x2⟶f′(x)<0x2<x⟶f′(x)>0
So, the answer :
f(x) increases on (−∞;−391−39586)∪(−391+39586;+∞)
f(x) decreases on (−391−39586;−391+39586)
b) Plot of y=f(x) is concave upwards if f′′(x)<0 .
Plot of y=f(x) is concave downwards if f′′(x)>0
f′′(x)=(f′(x))′=78x+2,f′′(x0)=0⟶x0=−391
So, ... upwards if (x<−391) , and ... downwards if (−391<x)
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