Question #280698

Let F(x)=∫

t−3

t

2+7

for − ∞ < x < ∞

x


(a) Find the value of x where F attains its minimum value.

(b) Find intervals over which F is only increasing or only decreasing.

(c) Find open intervals over which F is only concave up or only concave down.



1
Expert's answer
2022-01-04T08:05:24-0500
F(x)=0x(t3t2+7)dtF(x)=\displaystyle\int_{0}^x(t-3t^2+7)dt


By the Fundamental Theorem of Calculus


F(x)=x3x2+7F'(x)=x-3x^2+7

Find the critical points of F(x)F(x)

F(x)=0=>x3x2+7=0F'(x)=0=>x-3x^2+7=0

3x2x7=03x^2-x-7=0

D=(1)24(3)(7)=85D=(-1)^2-4(3)(-7)=85

x=1±852(3)x=\dfrac{1\pm\sqrt{85}}{2(3)}

x1=1856,x2=1+856x_1=\dfrac{1-\sqrt{85}}{6}, x_2=\dfrac{1+\sqrt{85}}{6}

If x<1856,F(x)<0,F(x)x<\dfrac{1-\sqrt{85}}{6}, F'(x)<0, F(x) decreases.

If 1856<x<1+856,F(x)>0,F(x)\dfrac{1-\sqrt{85}}{6}<x<\dfrac{1+\sqrt{85}}{6}, F'(x)>0, F(x) increases.

If x>1+856,F(x)<0,F(x)x>\dfrac{1+\sqrt{85}}{6}, F'(x)<0, F(x) decreases.


(a)The function F(x)F(x) has the local minimum at x=1856.x=\dfrac{1-\sqrt{85}}{6}.


(b) The function F(x)F(x) increases on

(1856,1+856).(\dfrac{1-\sqrt{85}}{6}, \dfrac{1+\sqrt{85}}{6}).

The function F(x)F(x) decreases on

(,1856)(1+856,)(-\infin, \dfrac{1-\sqrt{85}}{6})\cup(\dfrac{1+\sqrt{85}}{6}, \infin)

(c)


F(x)=(x3x2+7)=16xF''(x)=(x-3x^2+7)'=1-6x

Find the inflection point(s)


F(x)=0=>16x=0=>x=16F''(x)=0=>1-6x=0=>x=\dfrac{1}{6}

If x<16,F(x)>0,F(x)x<\dfrac{1}{6}, F''(x)>0, F(x) is concave up.

If x>16,F(x)<0,F(x)x>\dfrac{1}{6}, F''(x)<0, F(x) is concave down.

The function F(x)F(x) is only concave up on (,16).(-\infin, \dfrac{1}{6}).

The function F(x)F(x) is only concave down on (16,).(\dfrac{1}{6}, \infin).

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