How do I know which function is the upper boundary curve and which is the lower boundary curve. For example find the area between the curves e^x and x bounded on the sides x=0 and x=1. We can draw it and we know that e^x is the upper curve and x is the lower curve. Thus the area is ∫e^x-∫x. However, let's say I did not know how to draw the function, I could easily make the mistake and and solve Area = ∫x-∫e^x. The answer would be negative. Or say we had two very complicated functions, how would I know which is the upper and which is the lower boundary curve?