# Integration: Trig Substitution

When is th ebest time to use it and what are some good rules of thumb for it?

## Answers and Replies

Gold Member
MHB
Usually when you an integral in one of these three forms:

$$\int \sqrt{x^2+c^2}dx$$
$$\int \sqrt{x^2-c^2}dx$$
$$\int \sqrt{c^2-x^2}dx$$

Staff Emeritus
Gold Member
It's much more general than that, though. The quadratic forms in those integrands need not appear under a radical, and they need not appear in the "numerator line" of an expression (IOW, they can be on the bottom).

Homework Helper
Generally speaking you should remember that
cos2x= 1- sin2x
tan2x= sec2x- 1 and
sec2x= 1+ tan2x

Any time you have 1- x2, x2- 1, or 1+ x2 or can reduce to (as, for example 9- x2) you might consider using a trig substitution (unless, of course, something simpler works).

Homework Helper
Instead of the tangent-secans relation, you can also use the fact that $\cosh ^2 x - \sinh ^2 x = 1$. (cp the law with sin and cos, but here with a - instead of a +)

amcavoy
TD said:
Instead of the tangent-secans relation, you can also use the fact that $\cosh ^2 x - \sinh ^2 x = 1$. (cp the law with sin and cos, but here with a - instead of a +)

Yeah. At least for me, that has come up a lot in dealing with arc-length.

Homework Helper
Indeed, for example

Instead of the tangent-secans relation, you can also use the fact that $\cosh ^2 x - \sinh ^2 x = 1$. (cp the law with sin and cos, but here with a - instead of a +)