# Chapter 7 - Section 7.4 - Adding and Subtracting Rational Expressions with Different Denominators - Exercise Set - Page 515: 18

$\dfrac{-9}{25x^{2}-1}+\dfrac{7}{1-25x^{2}}=-\dfrac{16}{25x^{2}-1}$

#### Work Step by Step

$\dfrac{-9}{25x^{2}-1}+\dfrac{7}{1-25x^{2}}$ Change the sign of the numerator and the denominator of the second fraction: $\dfrac{-9}{25x^{2}-1}+\dfrac{7}{1-25x^{2}}=\dfrac{-9}{25x^{2}-1}+\dfrac{-7}{25x^{2}-1}=...$ Now we have to rational expressions with the same denominator, evaluate the sum by adding the numerators and using the same denominator. After that, simplify if possible: $...=\dfrac{-9-7}{25x^{2}-1}=-\dfrac{16}{25x^{2}-1}$

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