## Basic College Mathematics (9th Edition)

We can tell whether or not a proportion is true by cross-multiplying and checking for equality. For example, consider this proportion from the previous exercise: $\displaystyle \frac{30}{6}=\frac{45}{9}$ We cross-multiply: $30*9=45*6$ And check the equality: $270=270$ Since we got an identity, we know that the original proportion was true.