Tangent half-angle substitution is a technique primarily utilised in integral calculus.
The substitution[]
Differential[]
First, let Differentiate using the chain rule
- .
Consider the Pythagorean identity
Therefore,
Isolate ,
Sine[]
Consider the double angle identity,
Which may be written as,
Which in turn may be written as,
Recall that,
We can therefore rewrite the above as,
Therefore,
- .
Cosine[]
Consider the double angle identity,
.
Rewriting,
Writing in terms of t,
- .
Examples[]
Example 1[]
Let,
- .
Using,
- .
We may rewrite the integral as,
- .
- .
From there,
- .
Use the substitution,
- .
Therefore,
Isolate ,
So,
Example 2[]
Let
Multiplying through
Rearranging,
- .
First case,
Apply arctangent to both sides
Second case,
Again, apply arctangent to both sides,
So the two general solutions are,
Where is any integer.