1. **Stating the problem:** Solve the cubic equation $$3x^2 - 6 - x^3 = 0$$ for $x$.
2. **Rewrite the equation:** Rearrange terms to standard polynomial form:
$$-x^3 + 3x^2 - 6 = 0$$
Multiply both sides by $-1$ to simplify:
$$x^3 - 3x^2 + 6 = 0$$
3. **Try to find rational roots using the Rational Root Theorem:** Possible roots are factors of 6 (constant term) over factors of 1 (leading coefficient), i.e., $\pm1, \pm2, \pm3, \pm6$.
4. **Test $x=1$:**
$$1^3 - 3(1)^2 + 6 = 1 - 3 + 6 = 4 \neq 0$$
5. **Test $x=2$:**
$$2^3 - 3(2)^2 + 6 = 8 - 12 + 6 = 2 \neq 0$$
6. **Test $x=3$:**
$$3^3 - 3(3)^2 + 6 = 27 - 27 + 6 = 6 \neq 0$$
7. **Test $x=-1$:**
$$(-1)^3 - 3(-1)^2 + 6 = -1 - 3 + 6 = 2 \neq 0$$
8. **Test $x=-2$:**
$$(-2)^3 - 3(-2)^2 + 6 = -8 - 12 + 6 = -14 \neq 0$$
9. **Test $x=-3$:**
$$(-3)^3 - 3(-3)^2 + 6 = -27 - 27 + 6 = -48 \neq 0$$
10. **Test $x=-6$:**
$$(-6)^3 - 3(-6)^2 + 6 = -216 - 108 + 6 = -318 \neq 0$$
11. Since no rational roots are found, use the depressed cubic substitution or numerical methods.
12. **Rewrite original equation:**
$$-x^3 + 3x^2 - 6 = 0$$
Multiply both sides by $-1$:
$$x^3 - 3x^2 + 6 = 0$$
13. **Use substitution $x = y + \frac{b}{3a}$ to remove quadratic term:**
Here, $a=1$, $b=-3$, so
$$x = y + \frac{3}{3} = y + 1$$
14. Substitute into equation:
$$ (y+1)^3 - 3(y+1)^2 + 6 = 0$$
15. Expand:
$$ y^3 + 3y^2 + 3y + 1 - 3(y^2 + 2y + 1) + 6 = 0$$
16. Simplify:
$$ y^3 + 3y^2 + 3y + 1 - 3y^2 - 6y - 3 + 6 = 0$$
$$ y^3 + (3y^2 - 3y^2) + (3y - 6y) + (1 - 3 + 6) = 0$$
$$ y^3 - 3y + 4 = 0$$
17. **Solve depressed cubic:**
$$ y^3 - 3y + 4 = 0$$
18. Use the cubic formula or try to find roots by inspection.
19. Test $y=1$:
$$1 - 3 + 4 = 2 \neq 0$$
20. Test $y=-1$:
$$-1 + 3 + 4 = 6 \neq 0$$
21. Test $y=2$:
$$8 - 6 + 4 = 6 \neq 0$$
22. Test $y=-2$:
$$-8 + 6 + 4 = 2 \neq 0$$
23. Since no simple roots, use the cubic formula:
For equation $$y^3 + py + q = 0$$ with $p = -3$, $q = 4$.
Discriminant:
$$\Delta = \left(\frac{q}{2}\right)^2 + \left(\frac{p}{3}\right)^3 = \left(\frac{4}{2}\right)^2 + \left(\frac{-3}{3}\right)^3 = 2^2 + (-1)^3 = 4 - 1 = 3 > 0$$
24. Since $\Delta > 0$, one real root and two complex roots.
25. Real root:
$$ y = \sqrt[3]{-\frac{q}{2} + \sqrt{\Delta}} + \sqrt[3]{-\frac{q}{2} - \sqrt{\Delta}} = \sqrt[3]{-2 + \sqrt{3}} + \sqrt[3]{-2 - \sqrt{3}}$$
26. Approximate numerically:
$$ \sqrt{3} \approx 1.732$$
$$ y \approx \sqrt[3]{-0.268} + \sqrt[3]{-3.732} \approx -0.645 - 1.557 = -2.202$$
27. Recall substitution $x = y + 1$:
$$ x \approx -2.202 + 1 = -1.202$$
28. **Final answer:**
The equation has one real root approximately
$$ x \approx -1.202$$
and two complex roots (not calculated here).
Solve Cubic 3D293C
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