a/b - (na-1)/(nb+1)

This section deals with a very specific pattern of fraction subtraction that would be very difficult without knowing the formula. The problems appear in one of two forms:

1. $\frac{a}{b} - \frac{na - 1}{nb + 1} = \frac{a+b}{b(nb+1)}$

2. $\frac{a}{b} - \frac{na + 1}{nb - 1} = \frac{-(a+b)}{b(nb-1)}$

To solve, identify which formula applies by checking if the numerator and denominator of the second fraction are one more or less than a multiple ($n$) of the first fraction's numerator and denominator. The answer's numerator is simply the sum (or negative sum) of the numerator and denominator of the first fraction, and the denominator is the product of the two denominators.