If a planar curve in R 2 {\displaystyle \mathbb {R} ^{2}} is defined by the equation y = f ( x ) , {\displaystyle y=f(x),} where f {\displaystyle f} is continuously differentiable, then it is simply a special case of a parametric equation where x = t {\displaystyle x=t} and y = f ( t ) . {\displaystyle y=f(t).} The arc length is then given by: