# Derivative (max and min)

1. Dec 13, 2009

### naspek

Let f(x) = 5x^3 +4x + 8

(i) find f'(x)

(ii) Show that f(x) is increasing on ($$-\infty,\infty$$)

answer--> dont know how to prove it..

(iii)Hence find the minimum and maximum value of f(x) on the closed
interval [1,3]

2. Dec 13, 2009

### phsopher

A function is increasing if the derivative is greater than zero. If the function is increasing then what are its maximum and minimum values in a closed interval?

3. Dec 13, 2009

### naspek

should i search for critical points first?

4. Dec 13, 2009

### phsopher

If you like. But you can see directly from the expression of the derivative that it's never equal to zero.