Limits, Continuity & Differentiability
General
Grade 12
Question:
If <span class="math-inline">\(\lim_{x \to -1} \frac{x^2 - ax + b}{x - 1} = 5\)</span>, then <span class="math-inline">\(a + b\)</span> is equal to :-
-7
<span class="math-inline">\(2 - 4\)</span>
5
1
Step-by-Step Solution
Key Concept: Since the limit exists and is finite as x → -1, the denominator (x-1) evaluated at x = -1 gives -2 ≠ 0, so we can directly substitute x = -1 into the function without requiring the numerator to be zero.
Step 1: Evaluate the denominator at $x = -1$.
The denominator is $x - 1$.
Substituting $x = -1$ into the denominator yields:
$$(-1) - 1 = -2$$
Since the denominator is non-zero, the limit can be found by direct substitution.
Step 2: Substitute $x = -1$ into the expression.
Given that $\lim_{x \to -1} \frac{x^2 - ax + b}{x - 1} = 5$, we substitute $x = -1$ into the expression:
$$\frac{(-1)^2 - a(-1) + b}{(-1) - 1} = 5$$
Step 3: Simplify the expression.
$$ \frac{1 + a + b}{-2} = 5 $$
Step 4: Solve for $a + b$.
Multiply both sides by $-2$:
$$ 1 + a + b = 5 \times (-2) $$
$$ 1 + a + b = -10 $$
Subtract $1$ from both sides:
$$ a + b = -10 - 1 $$
$$ a + b = -11 $$
Correct Answer: 1