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Confused_111 · Nov 17, 2007, 03:05 AM

Hi there could you please help me with this, I don't know where to start after I differentiated it:

If f(x)=9+2x^2-x^3, f`(x)>0 for:
a) x<0
b) x>4/3
c) X<0 or x>4/3
d) 0<x<4/3
e) all x

galactus · Nov 17, 2007, 10:32 AM

I assume you found your derivative?

$f'(x)=-3x^{2}+4x>0$

$-x(3x-4)>0$

Now, solve the inequality. The solution is among your choices. Do not forget about switching sign direction when multiplying or dividing by a negative.