Q. What is the sum of \( \frac{7x}{x^2 – 4} \) and \( \frac{2}{x + 2} \)?

Answer

  1. Find a common denominator. Factor \( \frac{7x}{x^2 – 4} \) and \( \frac{2}{x + 2} \). This is the common denominator.
  2. Rewrite the second fraction. Multiply the numerator and denominator by (x – 2).\[ \frac{2(x – 2)}{x^2 – 4} \]
  3. Add the numerators. \[ \frac{7x + 2(x – 2)}{x^2 – 4} \]
  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:

  1. Write the expressions to be added:

    \(\frac{7x}{x^2 – 4}\) and \(\frac{2}{x + 2}\).

  2. Factor any quadratic denominators to find a common denominator. Factor \(x^2 – 4\) as a difference of squares:

    \(x^2 – 4 = (x – 2)(x + 2)\).

  3. 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:

    \(\frac{2}{x + 2} = \frac{2(x – 2)}{(x + 2)(x – 2)}\).

  4. Add the two fractions with the common denominator by adding their numerators:

    \(\frac{7x}{(x – 2)(x + 2)} + \frac{2(x – 2)}{(x – 2)(x + 2)} = \frac{7x + 2(x – 2)}{(x – 2)(x + 2)}\).

  5. 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

    \(\frac{9x – 4}{x^2 – 4}\).

  6. 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 \(x \neq \pm 2\)):

\(\frac{9x – 4}{x^2 – 4}\)

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FAQ

How do I add the rational expressions 7x/(x^2-4) and 2/(x+2)?

Factor denominators, find the least common denominator (LCD), rewrite each fraction with the LCD, add numerators, simplify, and state excluded values.

What is the LCD of x^2-4 and x+2?

Since x^2-4 = (x-2)(x+2), the LCD is (x-2)(x+2), equivalently x^2-4.

How do I rewrite 2/(x+2) with the LCD?

Multiply numerator and denominator by (x-2) to get 2(x-2)/((x+2)(x-2)).

What is the combined numerator after using the LCD?

Add numerators: 7x + 2(x-2) = 7x + 2x - 4 = 9x - 4.

What is the simplified sum?

(9x - 4)/(x^2 - 4), or (9x - 4)/((x-2)(x+2)). No further cancellation over integers.

Are there any values of x to exclude?

Yes. x = 2 and x = -2 are excluded because they make denominator zero.

Could any factor cancel between numerator and denominator?

Only if 9x - 4 shared factor (x-2) or (x+2). It does not, so no cancellation.

How can I check my result?

Substitute test value (not ±2), e.g. x = 0: left side 0/(−4) + 2/2 = 1; right side (−4)/(−4) = 1. Matches, so likely correct.
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