The minimum value of the expression $\dfrac{\sin^3\alpha + 6\sin^2\alpha + \sin\alpha + 2\cos^2\alpha - 8}{\sin\alpha - 1}$ is equal to:
Step-by-Step Solution
Key Concept: Simplification of trigonometric expressions by substitution and factoring
Step 1: Substitute $t = \sin\alpha$ to simplify the expression.
Since $\sin\alpha$ can take any value in $[-1, 1]$, let $t = \sin\alpha$ where $t \in [-1, 1]$ and $t \neq 1$ (to avoid division by zero). Also, note that $\cos^2\alpha = 1 - \sin^2\alpha = 1 - t^2$.
Step 2: Simplify the numerator by substituting $t$ and $\cos^2\alpha$.
The numerator is:
$$\sin^3\alpha + 6\sin^2\alpha + \sin\alpha + 2\cos^2\alpha - 8$$
Substituting $\sin\alpha = t$ and $\cos^2\alpha = 1 - t^2$:
$$t^3 + 6t^2 + t + 2(1-t^2) - 8$$
Expanding:
$$t^3 + 6t^2 + t + 2 - 2t^2 - 8 = t^3 + 4t^2 + t - 6$$
Step 3: Factor the numerator polynomial.
We need to factor $t^3 + 4t^2 + t - 6$. Testing $t = 1$:
$$1 + 4 + 1 - 6 = 0$$
So $(t-1)$ is a factor. Using polynomial division:
$$t^3 + 4t^2 + t - 6 = (t-1)(t^2 + 5t + 6)$$
Factoring the quadratic:
$$t^2 + 5t + 6 = (t+2)(t+3)$$
Therefore:
$$t^3 + 4t^2 + t - 6 = (t-1)(t+2)(t+3)$$
Step 4: Simplify the original expression by canceling the common factor.
The expression becomes:
$$\frac{(t-1)(t+2)(t+3)}{t-1} = (t+2)(t+3)$$
for $t \neq 1$. Expanding:
$$(t+2)(t+3) = t^2 + 5t + 6$$
Step 5: Find the minimum of the quadratic function on the domain $[-1, 1)$.
The quadratic $f(t) = t^2 + 5t + 6$ has its vertex at:
$$t = -\frac{5}{2} = -2.5$$
Since the vertex is at $t = -2.5$, which lies outside the domain $[-1, 1)$, the function is monotonically increasing on $[-1, 1)$.
Step 6: Evaluate the function at the left endpoint of the domain.
The minimum value occurs at $t = -1$:
$$f(-1) = (-1)^2 + 5(-1) + 6 = 1 - 5 + 6 = 2$$
Alternatively, using the factored form:
$$f(-1) = (-1+2)(-1+3) = (1)(2) = 2$$
**The minimum value of the given expression is $\boxed{2}$, which corresponds to Option 2.**
Correct Answer: 2