Cubic Curve
1. **Stating the problem:** We have a cubic function $$y = ax^3 + bx^2 + cx + d$$ that passes through points $(3,0)$ and $(0,6)$, and has a critical point at $(2,2)$.
2. **Using given points to form equations:**
- At $(3,0)$: $$a(3)^3 + b(3)^2 + c(3) + d = 0$$ simplifies to $$27a + 9b + 3c + d = 0$$
- At $(0,6)$: $$a(0)^3 + b(0)^2 + c(0) + d = 6$$ simplifies to $$d = 6$$
3. **Critical point condition:** The derivative is $$y' = 3ax^2 + 2bx + c$$.
At critical point $x=2$, $$y'(2) = 0$$, so
$$3a(2)^2 + 2b(2) + c = 0$$ which simplifies to $$12a + 4b + c = 0$$.
4. **Point $(2,2)$ lies on the curve:** Substitute into original function:
$$a(2)^3 + b(2)^2 + c(2) + d = 2$$
which is $$8a + 4b + 2c + d = 2$$.
5. **Substitute known $d=6$ into equations:**
From step 2: $$27a + 9b + 3c + 6 = 0$$ so $$27a + 9b + 3c = -6$$
From step 4: $$8a + 4b + 2c + 6 = 2$$ so $$8a + 4b + 2c = -4$$
6. **System of three equations with unknowns $a,b,c$:**
$$27a + 9b + 3c = -6$$
$$12a + 4b + c = 0$$
$$8a + 4b + 2c = -4$$
7. **Solve the system:**
From second equation: $$c = -12a - 4b$$.
Substitute into first:
$$27a + 9b + 3(-12a - 4b) = -6$$
$$27a + 9b - 36a - 12b = -6$$
$$-9a - 3b = -6
ightarrow 3a + b = 2$$
Substitute $c = -12a - 4b$ into third:
$$8a + 4b + 2(-12a - 4b) = -4$$
$$8a + 4b - 24a - 8b = -4$$
$$-16a - 4b = -4
ightarrow 4a + b = 1$$
8. **Solve for $a$ and $b$ from:**
$$3a + b = 2$$
$$4a + b = 1$$
Subtracting the second from the first:
$$(3a + b) - (4a + b) = 2 - 1$$
$$-a = 1
ightarrow a = -1$$
Substitute $a=-1$ into $3a + b = 2$:
$$3(-1) + b = 2
ightarrow -3 + b = 2
ightarrow b = 5$$
9. **Find $c$:**
$$c = -12a - 4b = -12(-1) - 4(5) = 12 - 20 = -8$$
10. **Recall $d=6$ from step 2.**
11. **Determine the type of critical point:**
Second derivative $$y'' = 6ax + 2b$$
At $x=2$:
$$y''(2) = 6(-1)(2) + 2(5) = -12 + 10 = -2 < 0$$
Since $y''(2)<0$, $(2,2)$ is a local maximum.
**Final answer:** $$a = -1, b = 5, c = -8, d = 6$$ and the critical point $(2,2)$ is a local maximum.
This corresponds to option C.