Evaluate the limit as x approaches 0 of (√(x+1) - √(2x+1)) / (√(3x+4) - √(2x+4)).
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Understand the Problem
The question asks to evaluate a limit as x approaches 0 of a function involving square roots. The function is a fraction with square root expressions in both the numerator and the denominator. To solve this, algebraic manipulation such as rationalization may be needed to avoid indeterminate forms.
Answer
-2
Answer for screen readers
-2
Steps to Solve
- Check for Indeterminate Form
First, plug in $x=0$ into the expression to see if we get an indeterminate form:
$$ \frac{\sqrt{0+1} - \sqrt{2(0)+1}}{\sqrt{3(0)+4} - \sqrt{2(0)+4}} = \frac{\sqrt{1} - \sqrt{1}}{\sqrt{4} - \sqrt{4}} = \frac{1-1}{2-2} = \frac{0}{0} $$
Since we have the indeterminate form $\frac{0}{0}$, we can use algebraic manipulation to simplify the expression.
- Rationalize the Numerator and Denominator
Multiply the numerator and denominator by their respective conjugates:
$$ \lim_{x \to 0} \frac{\sqrt{x+1} - \sqrt{2x+1}}{\sqrt{3x+4} - \sqrt{2x+4}} \cdot \frac{\sqrt{x+1} + \sqrt{2x+1}}{\sqrt{x+1} + \sqrt{2x+1}} \cdot \frac{\sqrt{3x+4} + \sqrt{2x+4}}{\sqrt{3x+4} + \sqrt{2x+4}} $$
- Simplify the Expression
Simplify the numerator: $(\sqrt{x+1} - \sqrt{2x+1})(\sqrt{x+1} + \sqrt{2x+1}) = (x+1) - (2x+1) = -x$. Simplify the denominator: $(\sqrt{3x+4} - \sqrt{2x+4})(\sqrt{3x+4} + \sqrt{2x+4}) = (3x+4) - (2x+4) = x$.
So we have:
$$ \lim_{x \to 0} \frac{-x}{x} \cdot \frac{\sqrt{3x+4} + \sqrt{2x+4}}{\sqrt{x+1} + \sqrt{2x+1}} $$
- Cancel $x$ and Evaluate the Limit
Cancel $x$ from the numerator and the denominator:
$$ \lim_{x \to 0} -1 \cdot \frac{\sqrt{3x+4} + \sqrt{2x+4}}{\sqrt{x+1} + \sqrt{2x+1}} $$
Now, plug in $x=0$:
$$ -1 \cdot \frac{\sqrt{3(0)+4} + \sqrt{2(0)+4}}{\sqrt{0+1} + \sqrt{2(0)+1}} = -1 \cdot \frac{\sqrt{4} + \sqrt{4}}{\sqrt{1} + \sqrt{1}} = -1 \cdot \frac{2+2}{1+1} = -1 \cdot \frac{4}{2} = -2 $$
-2
More Information
The limit evaluates to -2. This was found by rationalizing the numerator and denominator, simplifying the expression, and then evaluating the limit as x approaches 0.
Tips
A common mistake is not recognizing the indeterminate form and directly substituting $x=0$, which leads to an undefined expression. Failing to properly rationalize the numerator and denominator, or making algebraic errors during the simplification process, can also lead to an incorrect answer. Forgetting the negative sign in the numerator after rationalization is another possible mistake.
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