When we add or subtract two fractions, they must have the same denominator.
We use equivalent fractions to change them into two fractions with the same lowest common denominator.
We must first find a common multiple of b and d, which is bd.
Then we change each fraction into "bd" ths.
We multiply top and bottom of the first fraction by d.
We multiply top and bottom of the second fraction by b.
|=||a x d||+||c x b|
|b x d||d x b|
|=||ad + bc|