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

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