# Don't know how to approach this one

1. Oct 24, 2004

### Caldus

Let f be a function satisfying f'(x) = -3xf(x), f(1) = 4 and lim x->infinity f(x) = 0.

Evaluate integral of -3x*f(x)*dx between 1 and infinity.

Does this have to do with using comparison properties or something? I really don't know where to start. Thanks for any help.

2. Oct 25, 2004

### Muzza

$$\int -3xf(x) dx = \int f'(x) dx.$$

Surely you know how to integrate f'(x).