Interval question..

**elpermic** · October 23rd 2008, 05:11 PM

I don't get this..

Find all intervals of increasing&decreasing&all relative extrema. Find all intervals where f is concave up or down. Find all points of inflection.

f(x) = 6/(x^2+3)

I am lost on this, no idea how to solve it, I took the derivative and I get my x as sqrt(6i).

How do I get the concept of these problems? Sometimes I'm able to do them and other times I don't get the concept..

**skeeter** · October 23rd 2008, 05:48 PM

$f(x) = \frac{6}{x^2+3}$

$f'(x) = \frac{-12x}{(x^2+3)^2}$

$f'(x) = 0$ at $x = 0$

$f'(x) > 0$ for $x < 0$ ... $f(x)$ is increasing on the interval $(-\infty,0)$

$f'(x) < 0$ for $x > 0$ ... $f(x)$ is decreasing on the interval $(0, \infty)$

$f(x)$ has an absolute maximum of $y = 2$ at $x = 0$

$f''(x) = \frac{36(x^2-1)}{(x^2+3)^3}$

$f''(x) = 0$ at $x = 1$ and $x = -1$

$f''(x) > 0$ for $x < -1$ ... $f(x)$ is concave up

$f''(x) < 0$ for $-1 < x < 1$ ... $f(x)$ is concave down

$f''(x) > 0$ for $x > 1$ ... $f(x)$ is concave up

since $f''(x)$ changes sign at $x = 1$ and $x = -1$ , there are inflection points at those x-values.

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