For direct variation, use the equation y = kx, where k is the constant of proportionality. For inverse variation, use the equation y = k/x, again, with k as the constant of proportionality. In the direct variation, as one number increases, so does the other. It is also called direct proportion: they're the same thing.
It means that as x increases, y increases and as xx decreases, y decreases—and that the ratio between them always stays the same. The graph of the direct variation equation is a straight line through the origin.
In an inverse variation, it's precisely the opposite: as one number increases, the other decreases. It is also called the inverse proportion.
It means that as xx increases, yy decreases and as xx decreases, yy increases. The graph of the inverse variation equation is a hyperbola.
In a direct proportion, the ratio between matching quantities stays the same if they are divided. (They form equivalent fractions). In an indirect (or inverse) proportion, as one quantity increases, the other decreases. In an inverse proportion, the product of the matching quantities stays the same.