C(x) = 3x³ − 9x² − 243x + 1229
500 = 3x³ − 9x² − 243x + 1229
Subtract 500 on each side, as follows:
0 = 3x³ − 9x² − 243x + 729
(3x³ − 9x²) - (243x - 729) = 0
3x² (x - 3) - 243 (x - 3) = 0
(x - 3)(3x² - 243) = 0
x = 3, x = 9, x = -9; in the context of this problem, where x is the number
of units produced, negative values of x must be omitted, so x = 3 and x = 9
So we can say that if either 3 units or 9 units are produced the total cost
for the product will be at least $50000
[0, 3] U [9,∞)