Q. What is the sum of \( \frac{7x}{x^2 – 4} \) and \( \frac{2}{x + 2} \)?
Answer
- Find a common denominator. Factor \( \frac{7x}{x^2 – 4} \) and \( \frac{2}{x + 2} \). This is the common denominator.
- Rewrite the second fraction. Multiply the numerator and denominator by (x – 2).\[ \frac{2(x – 2)}{x^2 – 4} \]
- Add the numerators. \[ \frac{7x + 2(x – 2)}{x^2 – 4} \]
- Simplify. \[ \frac{9x – 4}{x^2 – 4} \]Valid for x not equal to 2 or -2.
Detailed Explanation
We want to add the two rational expressions:
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Write the expressions to be added:
\(\frac{7x}{x^2 – 4}\) and \(\frac{2}{x + 2}\).
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Factor any quadratic denominators to find a common denominator. Factor \(x^2 – 4\) as a difference of squares:
\(x^2 – 4 = (x – 2)(x + 2)\).
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Express both fractions with the common denominator \((x – 2)(x + 2)\). The first fraction already has that denominator:
\(\frac{7x}{(x – 2)(x + 2)}\).
The second fraction has denominator \(x + 2\), so multiply numerator and denominator by \((x – 2)\) to obtain the common denominator:
\(\frac{2}{x + 2} = \frac{2(x – 2)}{(x + 2)(x – 2)}\).
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Add the two fractions with the common denominator by adding their numerators:
\(\frac{7x}{(x – 2)(x + 2)} + \frac{2(x – 2)}{(x – 2)(x + 2)} = \frac{7x + 2(x – 2)}{(x – 2)(x + 2)}\).
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Expand and simplify the numerator:
\(7x + 2(x – 2) = 7x + 2x – 4 = 9x – 4\).
So the sum becomes
\(\frac{9x – 4}{(x – 2)(x + 2)}\), which can be written as
\(\frac{9x – 4}{x^2 – 4}\).
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State the domain restrictions: the original expressions are undefined when any denominator is zero, so \(x\) cannot equal \(2\) or \(-2\).
Final simplified sum (with domain restriction \(x \neq \pm 2\)):
\(\frac{9x – 4}{x^2 – 4}\)
FAQ
How do I add the rational expressions 7x/(x^2-4) and 2/(x+2)?
What is the LCD of x^2-4 and x+2?
How do I rewrite 2/(x+2) with the LCD?
What is the combined numerator after using the LCD?
What is the simplified sum?
Are there any values of x to exclude?
Could any factor cancel between numerator and denominator?
How can I check my result?
Pick the one that fits.
Math, Calculus, Geometry, etc.