“Rational Expressions” are simply algebraic fractions. Simplifying rational expressions means to make the fraction “look” more simple. The key is that the problem does not have an equal sign ( = ). Problems involving ( = ) prompt us to “solve”.
Simplifying rational expressions may involves reducing fractions, multiplying or dividing fractions, and adding or subtracting fractions. All of these operations on fractions will involve polynomials and will use factoring techniques.
(i.e. writing the fraction in “lowest terms”)
Now, apply this same procedure to fractions involving polynomials. For example, let’s write the following fraction in “lowest terms.”
… first we must factor both the numerator and the denominator
… now we observe that there is a common factor, namely
or … divide out the common factor and we get the fraction in “lowest terms”
Even though I removed the ( ), I cannot simplify any further. Removing the ( ) turns the factors into “terms” and “terms” cannot be divided!
The LCD is used to make equivalent fractions, to add or subtract fractions with unlike denominators, and to simplify complex fractions. The LCD is also useful in solving equations that contain fractions.
To find the LCD between two
polynomial denominators, factor each denominator completely. The
LCD is the product of the factors of the polynomials to the greatest
number of times that the factor is present in any one of the denominators.
LCD = (2)(3)(3)(5) = 90. This means that 90 is the least (smallest) number that can be used to make equivalent fractions of and so that they may be combined by addition or subtraction.
Notice that the (3) factor was used twice in the LCD since it was found the greatest number of times (twice) in 18.
Now, expand this understanding to polynomial denominators. Find the LCD between
and … factor the denominators and we get
and …observe the factors of the denominators
The LCD = y(y – 5)(y + 3). Notice we only use the (y – 5) once since it is only found once in each of the denominators.
Now suppose that we wanted to make equivalent
fraction using the LCD.
It is best to leave the denominators factored so that we can observe the factor that is necessary to transform the original fraction into the new fraction with the LCD.
The first fraction is missing the (y + 3) factor, so we need to multiply the original numerator of 8 with the factor (y + 3). Likewise, the second fraction is missing the (y) factor, so we need to multiply the original numerator of -2 with the factor (y).
... fractions now have a common denominator
Whenever you add or subtract fractions, you must have a common denominator. If we now wanted to add the fractions above we would simply add like terms in the numerators and retain the common denominator.
The views and
opinions expressed in this page are strictly those of Mary Lou Baker. This page was
The views and
opinions expressed in this page are strictly those of Mary Lou Baker.
This page was edited on 06-Nov-2007