Question #179838

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. 


1
Expert's answer
2021-04-29T16:54:55-0400

As I understood, the function is f(x)=13x3+x215x9f(x) = 13x^3+x^2-15x-9 .


a) Then, intervals in which f(x) is increasing \leftrightarrow f(x)>0f'(x) > 0 .

Intervals in which f(x) is decreasing \leftrightarrowf(x)<0f'(x) < 0 .

f(x)=39x2+2x15f'(x) = 39x^2+2x-15 \longrightarrow find nulls of f(x)f'(x)

x1,2=178(2±4+41539)=139(1±586),x1<x2x_{1,2} = \frac{1}{78}(-2 \pm \sqrt{4+4*15*39}) = \\ \frac{1}{39}(-1 \pm \sqrt{586}), x_1 < x_2

We have found null of quadratic trinomial, so since it's "branches" are directed up :

x<x1f(x)>0x1<x<x2f(x)<0x2<xf(x)>0x < x_1 \longrightarrow f'(x) >0 \\ x_1 < x < x_2 \longrightarrow f'(x) <0 \\ x_2 < x \longrightarrow f'(x) >0

So, the answer :

f(x) increases on (;13958639)(139+58639;+)(-\infty; -\frac{1}{39} - \frac{\sqrt{586}}{39}) \: \cup \: (-\frac{1}{39} + \frac{\sqrt{586}}{39}; +\infty)

f(x) decreases on (13958639;139+58639)(-\frac{1}{39}-\frac{\sqrt{586}}{39};-\frac{1}{39} + \frac{\sqrt{586}}{39})

b) Plot of y=f(x)y = f(x) is concave upwards if f(x)<0f''(x) < 0 .

Plot of y=f(x)y = f(x) is concave downwards if f(x)>0f''(x) > 0

f(x)=(f(x))=78x+2,f(x0)=0x0=139f''(x) = (f'(x))' = 78x+2, f''(x_0) = 0 \longrightarrow x_0 = -\frac{1}{39}

So, ... upwards if (x<139)(x < -\frac{1}{39}) , and ... downwards if (139<x)(-\frac{1}{39} < x)


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