# Homework Help: Integration of trig function

1. Jun 27, 2010

### hasan_researc

1. The problem statement, all variables and given/known data

What is the integral of tan x sec2x with respect to x?

2. Relevant equations

3. The attempt at a solution

I have no idea as to how I should proceed!

2. Jun 27, 2010

### hikaru1221

Hint: $$tanxsec^2xdx=\frac{sinxdx}{cos^3x}$$

3. Jun 27, 2010

### hasan_researc

How can that help? I have no idea!

Also, if we let u = tan x, then we get the limit of sin x as x tends to infinity, which is nonsense.

4. Jun 27, 2010

### hikaru1221

Another hint: sinxdx = d( ... )?

5. Jun 27, 2010

### HallsofIvy

As you were told in another thread, the "indefinite" integral is just the anti-derivative. It has nothing to do with a limit at infinity.
To integrate
$$\int \frac{sin x}{cos^3 x} dx$$
Let u= cos(x).

6. Jun 27, 2010

### Staff: Mentor

Even more direct: If u = tanx, du = sec2x dx. The indefinite integral has the form $\int u du$.

7. Jun 30, 2010

### hasan_researc

I don't understand what d(.....) actually means. I guess it's a clever way of using calculus that I'm not familiar with. But I have used the substitution u = cos x as follows.

$$u = \cos x & \Rightarrow du = - sin x dx \\ \int \frac{\sin x dx}{cos^{3} x} & = - \int\frac{1}{u^3} du \\ & = \frac{1}{2} u^{-2} + c \\ & = \frac{1}{2\cos^{2}x} + c$$

But if I use u = tanx I get the following.

$$u = \tan x & \Rightarrow du = sec^{2} x dx \\ \int tan x sec^{2} x dx & = - \intu du \\ & = \frac{1}{2} u^{2} + c \\ & = \frac{tan^{2} x}{2} + c$$

8. Jun 30, 2010

### hasan_researc

Sorry I made a silly mistake in my Latex code. The correction is:

$$\int tan x sec^{2} x dx & = \int u du \\$$

9. Jun 30, 2010

### hasan_researc

And I don't know how to break lines in my Latex code. Sorry for that!

10. Jun 30, 2010

### vela

Staff Emeritus
They're not contradictory. Use the identity tan2 x + 1 = sec2 x.

11. Jun 30, 2010

### hasan_researc

Ok, so

$$\frac{1}{2}\sec^{2} x + c \\ & = \frac{1}{2}\tan^{2} x + \frac{1}{2} + c \\$$

Therefore, the constant of integration resulting from my math is 1/2 + c, whereas the constant of integration in the other result is c. Should we not be worried abt that? Or is it simply an effect of the use of different substitutions at the start of the problem?

12. Jun 30, 2010

### Staff: Mentor

If you get two different answers from an indefinite integral, they can differ by only a constant. (1/2)sec^2(x) and (1/2)tan^2(x) differ by a constant, which is what vela was saying.