SAT Math : How to add rational expressions with a common denominator

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #51 : Expressions

Simplify the following rational expression: (9x - 2)/(x2) MINUS (6x - 8)/(x2)

Possible Answers:

\(\displaystyle \frac{12x-6}{x}\)

\(\displaystyle \frac{15x+6}{x}\)

\(\displaystyle \frac{3x+6}{x^{2}}\)

\(\displaystyle 3x-10\)

Correct answer:

\(\displaystyle \frac{3x+6}{x^{2}}\)

Explanation:

Since both expressions have a common denominator, x2, we can just recopy the denominator and focus on the numerators. We get (9x - 2) - (6x - 8). We must distribute the negative sign over the 6x - 8 expression which gives us 9x - 2 - 6x + 8 ( -2 minus a -8 gives a +6 since a negative and negative make a positive). The numerator is therefore 3x + 6.

Example Question #11 : Expressions

Simplify the following rational expression:

\(\displaystyle \frac{7x-18}{x^{2}}+\frac{6x-14}{x^{2}}\)

 

Possible Answers:

\(\displaystyle \frac{x-4}{x^{2}}\)

\(\displaystyle \frac{13x-4}{x^{2}}\)

\(\displaystyle \frac{13x-32}{x^{2}}\)

\(\displaystyle \frac{13x-28}{x^{2}}\)

\(\displaystyle \frac{x-32}{x^{2}}\)

Correct answer:

\(\displaystyle \frac{13x-32}{x^{2}}\)

Explanation:

Since both fractions in the expression have a common denominator of \(\displaystyle x^{2}\), we can combine like terms into a single numerator over the denominator:

\(\displaystyle \frac{7x-18}{x^{2}}+\frac{6x-14}{x^{2}}\)

\(\displaystyle =\frac{(7x-18)+(6x-14)}{x^{2}}\)

\(\displaystyle =\frac{13x-32}{x^{2}}\)

Example Question #2 : How To Add Rational Expressions With A Common Denominator

Simplify the following rational expression:

\(\displaystyle \frac{5x-5}{2x^{2}} + \frac{7x+9}{2x^{2}}\)

Possible Answers:

\(\displaystyle \frac{6x+2}{x^{2}}\)

\(\displaystyle \frac{12x+8}{2x^{2}}\)

\(\displaystyle \frac{6x+8}{2x^{2}}\)

\(\displaystyle \frac{8x+4}{2x^{2}}\)

\(\displaystyle \frac{6x+4}{2x^{2}}\)

Correct answer:

\(\displaystyle \frac{6x+2}{x^{2}}\)

Explanation:

Since both rational terms in the expression have the common denominator \(\displaystyle 2x^{2}\), combine the numerators and simplify like terms:

 

\(\displaystyle \frac{5x-5}{2x^{2}} + \frac{7x+9}{2x^{2}}\)

\(\displaystyle =\frac{(5x-5)+(7x+9)}{2x^{2}}\)

\(\displaystyle =\frac{12x+4}{2x^{2}}\)

\(\displaystyle =\frac{6x+2}{x^2}\)

Example Question #702 : Algebra

Simplify the following expression:

\(\displaystyle \frac{10x-9}{x^{3}}+\frac{11x+12}{x^{3}}\)

Possible Answers:

\(\displaystyle \frac{21x+3}{x^{3}}\)

\(\displaystyle \frac{3-x}{x^{3}}\)

\(\displaystyle \frac{21x-3}{x^3}\)

\(\displaystyle \frac{3-21x}{x^{3}}\)

\(\displaystyle \frac{x+3}{x^{3}}\)

Correct answer:

\(\displaystyle \frac{21x+3}{x^{3}}\)

Explanation:

Since both terms in the expression have the common denominator \(\displaystyle x^{3}\), combine the fractions and simplify the numerators:

\(\displaystyle \frac{10x-9}{x^{3}}+\frac{11x+12}{x^{3}}\)

\(\displaystyle =\frac{(10x-9)+(11x+12)}{x^{3}}\)

\(\displaystyle =\frac{21x+3}{x^{3}}\)

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