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Sketch synthesis: domain, intercepts, asymptotes, monotonicity, and concavity

Consider the function n(x)=(x+1)1/3n(x) = (x+1)^{1/3}, defined for all real numbers.

The function n(x)=(x+1)1/3n(x) = (x+1)^{1/3} has domain all real numbers and no vertical asymptotes. Using n′(x)=13(x+1)−2/3n'(x) = \frac{1}{3}(x+1)^{-2/3} and n′′(x)=−29(x+1)−5/3n''(x) = -\frac{2}{9}(x+1)^{-5/3}, determine the complete behavior of n(x)n(x) needed for a full curve sketch: the xx- and yy-intercepts, whether n(x)n(x) is increasing or decreasing on its domain, and the coordinates of any inflection point. State the inflection point's coordinates as your final answer.

Sketch synthesis: domain, intercepts, asymptotes, monotonicity, and concavity graph
Hint

Find where n(x)=0n(x)=0 and n(0)n(0) for intercepts. Check the sign of n′(x)n'(x) on its domain (note n′n' is undefined at x=−1x=-1 but nn itself is still defined there, so there's no vertical asymptote). Then check where n′′(x)n''(x) changes sign — that's the candidate inflection point, and confirm nn is defined and continuous there.

Step-by-step solution

Setting n(x)=0n(x)=0 gives (x+1)1/3=0(x+1)^{1/3}=0, so x=−1x=-1; thus the xx-intercept is (−1,0)(-1,0). Evaluating at x=0x=0 gives n(0)=11/3=1n(0) = 1^{1/3} = 1, so the yy-intercept is (0,1)(0,1). Since n′(x)=13(x+1)−2/3=13(x+1)2/3n'(x) = \frac{1}{3}(x+1)^{-2/3} = \frac{1}{3(x+1)^{2/3}} is positive for all x≠−1x \neq -1 (a square root of a real number under the cube gives a nonnegative denominator), n(x)n(x) is increasing on the entire real line, with a vertical tangent (not asymptote) at x=−1x=-1 since n(−1)n(-1) is defined and finite.

For concavity, n′′(x)=−29(x+1)−5/3=−29(x+1)5/3n''(x) = -\frac{2}{9}(x+1)^{-5/3} = -\frac{2}{9(x+1)^{5/3}}. For x<−1x < -1, (x+1)5/3<0(x+1)^{5/3} < 0, making n′′(x)>0n''(x) > 0 (concave up); for x>−1x > -1, (x+1)5/3>0(x+1)^{5/3} > 0, making n′′(x)<0n''(x) < 0 (concave down). Since concavity switches sign across x=−1x=-1 and n(−1)=0n(-1)=0 is defined, the point (−1,0)(-1, 0) is an inflection point — matching the same location as the xx-intercept.

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