The 'Cross Multiply' Fraction Hack!

Struggling to add fractions with different denominators? What if I told you there's a super-fast way to do it?

Subject: algebra • Classes: 6–12 • Difficulty: basic (6-8)

The Trick

Adding fractions like $\frac{a}{b} + \frac{c}{d}$ seems tricky. The trick is to 'cross multiply' and add the results over the product of the denominators. So, $\frac{a}{b} + \frac{c}{d} = \frac{ad + bc}{bd}$. Why? Because we're finding a common denominator (bd) by multiplying each fraction by a form of 1: $\frac{a}{b} * \frac{d}{d} + \frac{c}{d} * \frac{b}{b}$

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