How do you find intercepts, extrema, points of inflections, asymptotes and graph h(x)=xsqrt(9-x^2)?

1 Answer
Aug 22, 2017

Please see below.

Explanation:

Domain : [-3,3]

Intercepts
y intercept h(0) = 0

x intercepts: -3, 0, and 3
(Solve h(x) = 0)

Asymptotes
The function has no horizontal asymptotes. (The domain is bouinded.) and no vertical asymptotes (there is no division).

h'(x) = (9-2x^2)/sqrt(9-x^2)

Is undefined only at the endpoints of the domain and is 0 at x = +-3/sqrt2

It is worth noting that as x approaches the endpoints of the domain, +-3, the derivative goes to +-oo So the tangent line becomes vertical.

Local extrema and increasing/decreasing
On [-3,-3/sqrt2), we have h'(x) < 0 so h is decreasing.
On (-3/sqrt2,0), we have h'(x) > 0 so h is increasing.

And h(-3/sqrt2) = -9/2 is a local minimum.

On (0,3/sqrt2), we have h'(x) > 0 so h is increasing.
On (3/sqrt2,3], we have h'(x) ><0 so h is decreasing.

And h(3/sqrt2) = 9/2 is a local maximum.

Concavity and inflection points

h''(x) = (x(2x^2-27))/(9-x^2)^(3/2)

Is undefined only at the endpoints of the domain and is 0 at x = 0 (The expression is also 0 at x = +-sqrt(27/2), but those are outside the domain of h.)

On [-3,0), we have h''(x) > 0 so the graph of h is concave upwards (convex).

On [-3,0), we have h''(x) < 0 so the graph of h is concave downwards (concave).

The point (0,0) is the only inflection point.

The graph is:

graph{xsqrt(9-x^2) [-9.23, 13.27, -5.625, 5.625]}