Chapter 11: Problem 75
Divide. $$\frac{y^{2}-y-56}{y^{2}+8 y+7} \div \frac{y^{2}-13 y+40}{y^{2}-4 y-5}$$
Short Answer
Expert verified
\[ \frac{y+5}{y-5} \]
Step by step solution
01
Factoring the Quadratics
First, each quadratic in the numerators and denominators needs to be factored: \[ \frac{(y-8)(y+7)}{(y+7)(y+1)} \div \frac{(y-8)(y-5)}{(y+1)(y+5)} \]
02
Convert Division to Multiplication
Then, rewrite the division as multiplication by taking the reciprocal of the second fraction: \[ \frac{(y-8)(y+7)}{(y+7)(y+1)} \times \frac{(y+1)(y+5)}{(y-8)(y-5)} \]
03
Simplification
Now common factors in the numerator and denominator can be canceled out. Cancel \(y-8\), \(y+7\), and \(y+1\) from top and bottom to simplify: \[ \frac{1}{1} \times \frac{y+5}{y-5} \]
04
Final Multiplication
Finally, because the first fraction has simplified to 1, the answer is just the second fraction: \[ \frac{y+5}{y-5} \]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Factoring Quadratics
In this concept, we focus on breaking down quadratic equations into factors. A quadratic polynomial typically takes the form of \(ax^2 + bx + c\). Factoring is essentially finding two binomials that multiply to give you the original polynomial. For the problem at hand, you have several quadratics to factor.
- \(y^2 - y - 56\) turns into \((y-8)(y+7)\).
- \(y^2 + 8y + 7\) becomes \((y+7)(y+1)\).
- \(y^2 - 13y + 40\) is factored into \((y-8)(y-5)\).
- \(y^2 - 4y - 5\) results in \((y+1)(y+5)\).
Fraction Multiplication
Once you have the quadratics factored, it's time to multiply fractions, a crucial step in solving complex rational expressions. Normally, to multiply two fractions, you multiply the numerators and then the denominators, resulting in a new fraction. But, dividing fractions requires an additional twist.Consider the fraction division from our exercise. Instead of dividing directly, we turn the division problem into a multiplication problem by using the reciprocal of the divisor. So, \(\frac{a}{b} \div \frac{c}{d}\) becomes \(\frac{a}{b} \times \frac{d}{c}\). For the exercise,
- Switch \(\frac{(y-8)(y-5)}{(y+1)(y+5)}\) with its reciprocal \(\frac{(y+1)(y+5)}{(y-8)(y-5)}\)
Simplification
Simplification involves cancelling out common factors in the numerator and the denominator to reduce the expression to its simplest form. It's much like removing common terms in a fraction, just tailored to polynomials.After setting up the multiplication, observe which terms appear in both the numerator and denominator. For our division problem, note the factors:
- Cancel out \((y-8)\), \((y+7)\), and \((y+1)\) common to both the fractions.
Reciprocal of a Fraction
Finally, let's explore the concept of reciprocals, a fundamental technique in fraction division. The reciprocal of a fraction \(\frac{a}{b}\) is simply \(\frac{b}{a}\). This form moves the denominator to the numerator position, and vice versa. While generally useful in fraction multiplication, it is indispensable when dividing fractions. The key step in transforming a division problem into a multiplication one is to take the reciprocal of the fraction that is being divided. For the given problem, the second fraction, \(\frac{(y-8)(y-5)}{(y+1)(y+5)}\), was flipped to \(\frac{(y+1)(y+5)}{(y-8)(y-5)}\) allowing for multiplication to proceed. With this approach, it not only solves the problem but illustrates the practical use of reciprocals in simplifying difficult polynomial divisions.