Hi,

I have revenue:

$\displaystyle r(x)=100x-.0001x^2$

and cost:

$\displaystyle c(x)=360+80x+.002x^2+.00001x^3$

I must maximise profit.

profit is: $\displaystyle r(x)-c(x)=-.00001x^3-.0021x^2+20x-360$

$\displaystyle p'(x)=-.00003x^2-.0042x+20$.

The answer is 749 units.

I cannot seem to factor p' to get this result.

I have tried completing the square.

$\displaystyle x^2+140x-\frac{20}{.00003}=0$

$\displaystyle (x+70)^2=70^2+\frac{20}{.00003}$

$\displaystyle x=-70\pm \sqrt{70^2+\frac{20}{.00003}}$

Calculating that out doesn't get me near 749.

I make it $\displaystyle x\approx{3431.3998}$

Thanks

Craig.