[Math] Area between two curves that do not intersect

areacalculus

the curves are $x^2 = 4y$ and $x^2=4y-4$
these are just the same parabolas but the other one is shifted up by one unit.

I have been thinking of 3 possibilities that might be the answer.

  1. The area is equal to infinite sq. units

  2. The area is equal zero

  3. The area is undefined

The answer I have concluded that is probably the most correct is that the area is undefined because:

  1. The area is not enclosed by the two curves

  2. Infinity is not a number

  3. The area is definitely not zero since the curves are not overlapping

So my question is if I answered this correctly.

Best Answer

You can rewrite this as an improper integrals:

$$\int_{-\infty}^\infty \left({x^2 + 4\over 4} - {x^2\over 4}\right) \,\mathrm dx = \int_{-\infty}^\infty 1\,\mathrm dx$$

It becomes obvious this does not converge thus the area is not finite:

$$\lim_{b\to\infty} x \,\Big|^b_{-b} \rightarrow \infty - (-\infty) = \infty$$

The notion that the area is undefined because the curves do not cross is wrong. Consider the Gaussian Integral which is between the functions $f(x) = e^{-x^2}$ and $g(x) = 0$. They do not cross, yet the integral from negative infinity to positive infinity is finite:

$$\int_{-\infty}^\infty e^{-x^2} \,\mathrm dx = \sqrt{\pi}$$

Note that $g(x) = 0$ is technically a curve. Mathematically speaking, a curve is a generalization of a line.

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