For brevity, formula_11 is often written as formula_12, with formula_13 being the Greek letter Delta, meaning 'change in'. The slope of a linear equation is constant, meaning that the steepness is the same everywhere. However, many graphs, for instance formula_14, vary in their steepness. This means that you can no longer pick any two arbitrary points and compute the slope. Instead, the slope of the graph can be computed by considering the tangent line—a line that 'just touches' a particular point. The slope of a curve at a particular point is equal to the slope of the tangent to that point. For example, formula_14 has a slope of formula_16 at formula_17 because the slope of the tangent line to that point is equal to formula_16: