This continues Area between functions to look at areas between curves where x is a function of y.
For a contiguous enclosed region bounded by functions x=f(y) and x=g(y), such that
y1<y2, AND
g(y)≥f(y) for all y∈[y1,y2]
or alternatively, each horizontal line through the area intersects x=f(y) on the left and x=g(y) on the right.
Then the bounded region has area
area=∫y1y2g(y)−f(y)dy
So instead of top curve minus bottom curve, it is now right curve minus left curve.
Example: Find the area enclosed between x=y2 and y=x−6.
The intersection points are (4,−2) and (9,3) so y1=−2 and y2=3. See sketch of the curves . Note that each horizontal line in the enclosed region meets f(y) on the left, and g(y) on the right. The area is
area =∫−23y+6−y2dy=[21y2+6y−31y3]−23=21(9−4)+6(3+2)−31(27+8)=25+30−335=2+21+30−12+31=20+65■
In Area between functions, we found the area enclosed between x=y2 and y=x−6, by breaking into two regions.
Similarly, the signed area between a function of y and the y-axis is
area to the left of the curve =∫y1y2f(y)dy
as opposed to
area under the curve =∫x1x2f(x)dx
Always quickly sketch the functions or curves using function transformations to visualize the area.