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Find the LCD of the following expressions. Write the LCD in factored form.

[tex]\[
\frac{4}{n^2 + 5n - 36} \quad \text{and} \quad \frac{7n}{n^2 - 7n + 12}
\][/tex]

LCD: [tex]\(\square\)[/tex]


Sagot :

To find the Least Common Denominator (LCD) of the given expressions:

[tex]\[ \frac{4}{n^2 + 5n - 36} \][/tex]
[tex]\[ \frac{7n}{n^2 - 7n + 12} \][/tex]

we follow these steps:

1. Factorize each denominator separately.

First denominator: [tex]\( n^2 + 5n - 36 \)[/tex]

To factorize this quadratic expression, we need to find two numbers that multiply to [tex]\(-36\)[/tex] (the constant term) and add to [tex]\(5\)[/tex] (the coefficient of [tex]\(n\)[/tex]).

These numbers are [tex]\(9\)[/tex] and [tex]\(-4\)[/tex]:

[tex]\[ n^2 + 5n - 36 = (n + 9)(n - 4) \][/tex]

So, the first denominator factors as:

[tex]\[ n^2 + 5n - 36 = (n - 4)(n + 9) \][/tex]

Second denominator: [tex]\( n^2 - 7n + 12 \)[/tex]

To factorize this quadratic expression, we need to find two numbers that multiply to [tex]\(12\)[/tex] (the constant term) and add to [tex]\(-7\)[/tex] (the coefficient of [tex]\(n\)[/tex]).

These numbers are [tex]\(-3\)[/tex] and [tex]\(-4\)[/tex]:

[tex]\[ n^2 - 7n + 12 = (n - 3)(n - 4) \][/tex]

So, the second denominator factors as:

[tex]\[ n^2 - 7n + 12 = (n - 4)(n - 3) \][/tex]

2. Identify all unique factors from both denominators.

For the denominators:

[tex]\[ n^2 + 5n - 36 = (n - 4)(n + 9) \][/tex]

and

[tex]\[ n^2 - 7n + 12 = (n - 4)(n - 3) \][/tex]

The unique factors are:
- [tex]\( (n - 4) \)[/tex]
- [tex]\( (n + 9) \)[/tex]
- [tex]\( (n - 3) \)[/tex]

3. Construct the Least Common Denominator (LCD).

The LCD is the product of the highest powers of all unique factors identified. Since we have:

[tex]\[ n^2 + 5n - 36 = (n - 4)(n + 9) \][/tex]

and

[tex]\[ n^2 - 7n + 12 = (n - 4)(n - 3) \][/tex]

The LCD must include each unique factor, appearing as many times as it does in any one denominator.

Therefore, we include each factor [tex]\( (n - 4) \)[/tex], [tex]\( (n + 9) \)[/tex], and [tex]\( (n - 3) \)[/tex]:

[tex]\[ \text{LCD} = (n - 4)(n - 3)(n + 9) \][/tex]

In conclusion, the Least Common Denominator (LCD) of the given expressions is:

[tex]\[ \boxed{(n - 4)(n - 3)(n + 9)} \][/tex]