Let , then .
Substituting gives
Finally
We compute integrals involving powers and products of trigonometric functions.
The following are the Pythagorean Trigonometric Identities (named for Pythagoras of Samos) which hold for all angles, , in the domains of the functions involved: and
Next, we have the half-angle formulas:
and
We will find the half-angle formulas useful for integrating even powers of sine and cosine.
In this section, we compute integrals of the form The method we use depends on whether and are even or odd.
In this case, the technique involves preparing and performing a -substitution. The key is to use the Pythagorean Identity to convert sines to cosines or vice-versa.
In the next example, we need to prepare for the -substitution.
Prepare for substitution by rewriting the odd power of cosine.
Rewrite the original integral. Make the substitution , .
Rewrite the odd power of cosine as
Next, let so that .
Substituting gives
Finally
Rewrite the odd power of sine as:
Next, let so that .
Substituting gives
Finally
Rewrite the odd power of cosine as:
Next, let so that .
Substituting gives
Finally
Rewrite the odd power of cosine as:
Next, let so that .
Substituting gives
Finally
Prepare for substitution by rewriting the odd power of cosine.
Rewrite the original integral.
Make the substitution .
Rewrite the odd power of sine as:
Next, let so that .
Substituting gives
Finally
Rewrite the odd power of cosine as:
Next, let so that .
Substituting gives
Finally
Rewrite the odd power of sine as:
Next, let so that .
Substituting gives
Finally
In the next example, both powers are odd. In this case, we have to choose which one to rewrite.
Prepare for substitution by rewriting the smaller odd power.
Rewrite the original integral.
Make the substitution .
Which trig function should be rewritten?
Rewrite the odd power of sine as:
Next, let so that .
Substituting gives
Finally
Which trig function should be rewritten?
Rewrite the odd power of cosine as:
Next, let so that .
Substituting gives
Finally
In this section, we compute integrals of the form where and are both
even.
We will use the half-angle formulas
Use a half angle formula to rewrite:
Use a half-angle formula again to obtain,
Finally,
Use the half-angle formulas to rewrite:
Use a half-angle formula again to obtain,
Finally,
In this section, we compute integrals of the form where either is even or is odd. We will use the identity to prepare a -substitution.
Prepare for -substitution by rewriting the even power of secant.
Rewrite the original integral.
Make the substitution .
Rewrite the even power of secant as
Next, let so that .
Substituting gives
Finally
Rewrite the even power of secant as
Next, let so that .
Substituting gives
Finally
Rewrite the even power of secant as
Next, let so that .
Substituting gives
Finally
Compute With an odd power of tangent, we prepare the substitution by rewriting the integral as Next we perform the substitution:
Next, convert tangents to secants, using
The integral becomes
Now we are ready to substitute,
Let , then
Substituting gives
Finally
Let , then
Substituting gives
Finally
Furthermore, IBP also gave a reduction formula for higher powers of : This formula is particularly useful if the power is odd, since if is odd, is also odd. Therefore, if we repeat the formula enough times, we will eventually be left with which we have just solved.