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 P: 26 Okay, thanks. But in this notation $\int^{b}_{a}{f(x).dx}$ I know it means area under the curve on interval [a,b] what if I take dx<0 how can I explain it? I mean I can take limit (lim $\frac{Δy}{Δx}$ as x approximates either side of x-axis (Δx→0$^{+}$ and Δx→0$^{-}$) I realize that if dx>0 or dx<0 at derivative of a function but I'm not sure about integral??