Consider the integral

At first glance, we might try the substitution u = 9-x², but this will actually make the integral even more complicated!

Let's try a different approach:

The radical

For this triangle, sinq = ^x/₃, suggesting the substitution x = 3sinq. Then q = arcsin(^x/₃), where we specify -p/2 £ q £ p/2. Note that dx = 3cosq dq and that

With this change of variables,

Caution!

• The sketch of the triangle is very useful for determining what substitution should be made. Note, though, that the sketch only has meaning for x > 0 and q > 0.

• It is important to be careful about how the angle q is defined. With the restrictions on q mentioned in the examples here, we avoid sign difficulties even when x < 0.

There are two other trigonometric substitutions useful in integrals with different forms:

Example

Let's evaluate

The radical

For this triangle, secq = ^x/₂, we we will try the substitution x = 2secq. Then q = sec^-1 (^x/₂), where we specify 0 £ q < p/2 or p £ q < 3p/2. Note that dx = 2secqtanq dq and that

Then

But we see from the sketch that

We may also use a trigonometric substitution to evaluate a definite integral, as long as care is taken in working with the limits of integration:

Example

We will evaluate

The factor (1+x²) suggests a triangle with base of length 1 and height x:

For this triangle, tanq = x, so we will try the substitution x = tanq. Then q = tan^-1(x), where we specify -p/2 < q < p/2. Here, dx = sec²q dq. Also,

Then

There is often more than one way to solve a particular integral. A trigonometric substitution will not always be necessary, even when the types of factors seen above appear. With practice, you will gain insight into what kind of substitution will work best for a particular integral.

Trigonometric substitutions are often useful for integrals containing factors of the form

The exact substitution used depends on the form of the integral:

(a2-x2)n	(x2+a2)n	(x2-a2)n
x = asinq	x = atanq	x = asecq
dx = acosq	dx = asec2q	dx = asecqtanq
______ Ö a2-x2   = acosq	______ Ö x2+a2   = asecq	______ Ö x2-a2   = atanq