Finding the area between two curves, usually given by two explicit functions, is often useful in calculus.
In general the rule for finding the area between two curves is
If f(x) is the upper function and g(x) is the lower function
This is true whether the functions are in the first quadrant or not.
Area between two curves
Suppose we are given two functions y1=f(x) and y2=g(x) and we want to find the area between them on the interval [a,b]. Also assume that f(x)≥ g(x) for all x on the interval [a,b]. Begin by partitioning the interval [a,b] into n equal subintervals each having a length of Δx=(b-a)/n. Next choose any point in each subinterval, xi*. Now we can 'create' rectangles on each interval. At the point xi*, the height of each rectangle is f(xi*)-g(xi*) and the width is Δx. Thus the area of each rectangle is [f(xi*)-g(xi*)]Δx. An approximation of the area, A, between the two curves is
Now we take the limit as n approaches infinity and get
which gives the exact area. Recalling the definition of the definite integral we notice that
This formula of finding the area between two curves is sometimes known as applying integration with respect to the x-axis since the rectangles used to approximate the area have their bases lying parallel to the x-axis. It will be most useful when the two functions are of the form y1=f(x) and y2=g(x). Sometimes however, one may find it simpler to integrate with respect to the y-axis. This occurs when integrating with respect to the x-axis would result in more than one integral to be evaluated. These functions take the form x1=f(y) and x2=g(y) on the interval [c,d]. Note that [c,d] are values of y. The derivation of this case is completely identical. Similar to before, we will assume that f(y)≥ g(y) for all y on [c,d]. Now, as before we can divide the interval into n subintervals and create rectangles to approximate the area between f(y) and g(y). It may be useful to picture each rectangle having their 'width', Δy, parallel to the y-axis and 'height', f(yi*)-g(yi*) at the point yi*, parallel to the x-axis. Following from the work above we may reason that an approximation of the area, A, between the two curves is
As before, we take the limit as n approaches infinity to arrive at
which is nothing more than a definite integral, so
Regardless of the form of the functions, we basically use the same formula.