(a) Domain:
The function is defined for all x except where the denominator is zero. The denominator (x+1)2 is zero when x+1=0, so x=−1. Therefore, the domain of f is all real numbers except x=−1. We can write this as x∈(−∞,−1)∪(−1,∞). (b) Intercepts:
To find the x-intercepts, we set f(x)=0. (x+1)2x3=0 implies x3=0, so x=0. Thus, the x-intercept is (0,0). To find the y-intercepts, we set x=0. f(0)=(0+1)203=10=0. Thus, the y-intercept is (0,0). (c) First and second derivatives:
We use the quotient rule to find the first derivative f′(x). The quotient rule is dxd(vu)=v2vdxdu−udxdv. Here, u=x3 and v=(x+1)2. So, dxdu=3x2 and dxdv=2(x+1). f′(x)=(x+1)4(x+1)2(3x2)−x3(2(x+1))=(x+1)4(x+1)(3x2(x+1)−2x3)=(x+1)33x3+3x2−2x3=(x+1)3x3+3x2=(x+1)3x2(x+3) Now, we find the second derivative f′′(x). Again, we use the quotient rule with u=x3+3x2 and v=(x+1)3. Then dxdu=3x2+6x and dxdv=3(x+1)2. f′′(x)=(x+1)6(x+1)3(3x2+6x)−(x3+3x2)(3(x+1)2)=(x+1)6(x+1)2[(x+1)(3x2+6x)−(x3+3x2)(3)]=(x+1)43x3+6x2+3x2+6x−3x3−9x2=(x+1)46x (d) Critical points:
Critical points occur when f′(x)=0 or f′(x) is undefined. f′(x)=(x+1)3x2(x+3)=0 when x2(x+3)=0, which gives x=0 and x=−3. f′(x) is undefined when (x+1)3=0, which means x=−1. Since x=−1 is not in the domain of f, we only consider x=0 and x=−3. Thus, the critical points are x=0 and x=−3. The critical points are (−3,f(−3))=(−3,(−3+1)2(−3)3)=(−3,4−27) and (0,f(0))=(0,0). (e) Intervals of increase and decrease:
We analyze the sign of f′(x)=(x+1)3x2(x+3). - For x<−3, x2>0, x+3<0, and (x+1)3<0, so f′(x)=(−)(+)(−)=(+)>0. - For −3<x<−1, x2>0, x+3>0, and (x+1)3<0, so f′(x)=(−)(+)(+)=(−)<0. - For −1<x<0, x2>0, x+3>0, and (x+1)3>0, so f′(x)=(+)(+)(+)=(+)>0. - For x>0, x2>0, x+3>0, and (x+1)3>0, so f′(x)=(+)(+)(+)=(+)>0. Thus, f is increasing on (−∞,−3) and (−1,∞), and decreasing on (−3,−1). (f) Points of inflection:
Points of inflection occur when f′′(x)=0 or f′′(x) is undefined and the concavity changes sign. f′′(x)=(x+1)46x=0 when 6x=0, so x=0. f′′(x) is undefined when (x+1)4=0, or x=−1, which is not in the domain. We check the sign of f′′(x) around x=0: - For x<0 (and x>−1), f′′(x)<0, so the graph is concave down. - For x>0, f′′(x)>0, so the graph is concave up. Since the concavity changes at x=0, (0,0) is a point of inflection. (g) Asymptotes:
Vertical asymptotes occur where the function is undefined. Thus there is a vertical asymptote at x=−1. To find horizontal asymptotes, we consider the limit as x approaches infinity. limx→∞(x+1)2x3=limx→∞x2+2x+1x3=limx→∞1+2/x+1/x2x=∞. limx→−∞(x+1)2x3=limx→−∞x2+2x+1x3=limx→−∞1+2/x+1/x2x=−∞. Since the limits are infinite, there are no horizontal asymptotes.
To check for slant asymptotes, we perform polynomial division:
x2+2x+1x3=x−2+x2+2x+13x+2. As x→±∞, x2+2x+13x+2→0. Thus, there is a slant asymptote at y=x−2. (h) Plot key points and sketch the curve.
Key points:
Intercept: (0,0)
Critical point: (-3, -27/4) = (-3, -6.75)
Inflection point: (0,0)
Asymptotes: x=-1, y=x-2
Increase: (−∞,−3),(−1,∞) Decrease: (−3,−1)