Faraday effect

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In mathematics, a quadratic integral is an integral of the form

dxa+bx+cx2.

It can be evaluated by completing the square in the denominator.

dxa+bx+cx2=1cdx(x+b2c)2+(acb24c2).

Positive-discriminant case

Assume that the discriminant q = b2 − 4ac is positive. In that case, define u and A by

u=x+b2c,

and

A2=acb24c2=14c2(4acb2).

The quadratic integral can now be written as

dxa+bx+cx2=1cduu2A2=1cdu(u+A)(uA).

The partial fraction decomposition

1(u+A)(uA)=12A(1uA1u+A)

allows us to evaluate the integral:

1cdu(u+A)(uA)=12Acln(uAu+A)+constant.

The final result for the original integral, under the assumption that q > 0, is

dxa+bx+cx2=1qln(2cx+bq2cx+b+q)+constant, where q=b24ac.

Negative-discriminant case

This (hastily written) section may need attention.

In case the discriminant q = b2 − 4ac is negative, the second term in the denominator in

dxa+bx+cx2=1cdx(x+b2c)2+(acb24c2).

is positive. Then the integral becomes

1cduu2+A2=1cAdu/A(u/A)2+1=1cAdww2+1=1cAarctan(w)+constant=1cAarctan(uA)+constant=1cacb24c2arctan(x+b2cacb24c2)+constant=24acb2arctan(2cx+b4acb2)+constant.

References

  • Weisstein, Eric W. "Quadratic Integral." From MathWorld--A Wolfram Web Resource, wherein the following is referenced:
  • Gradshteyn, I. S. and Ryzhik, I. M. Tables of Integrals, Series, and Products, 6th ed. San Diego, CA: Academic Press, 2000.