Question

In Calculus, when you are finding the area between curves, how do you know when you should use horizontal rectangles instead of vertical?

Answer 1

you can use either but make sure that it cuts both curves and not one curve.
sometimes you are forced to choose one over the other as the vertical/horizontal line must intersect both curves. hope it answers your question

Answer 2

Why in this graph would you elect to use horizontal rectangles. I don't quite get it still

Answer 3

yes you would use horizontal as any horizontal line you take crosses both lines, whereas if you took a vertical line then at some stage it does not cut through both lines. i will draw to illustrate this