Solving the Integral of cos(2x)

Steps to Solve

We are interested in finding the integral of cos(2x). To do this, we are going to make use of integration by substitution. Integration by substitution is an integration method that can be used on integrals that are in the form ∫ f(g(x)) dx, and can be put in the following form:

∫ f(g(x)) ⋅ g ‘ (x) dx

When we have an integral in this form, we can make a u-substitution where u = g(x), so du = g ‘ (x) dx. By making this substitution, we end up with the following:

∫ f(g(x)) ⋅ g ‘ (x) dx = ∫ f(u) du

Then, we can find the integral of f(u) and plug g(x) back in for u to get the integral. Hmmm…that sounds a little confusing. Let’s see if writing this process out in steps makes it a bit more clear.

To find ∫ f(g(x)) ⋅ g ‘ (x) dx, we follow these steps.

1. Let u = g(x). Then du = g ‘ (x) dx, and dx = (1/g ‘ (x))du.
2. Plug these values into the integral appropriately to get it in the form ∫ f(u) du or ∫ a ⋅ f(u) du, where a is a constant.
3. Evaluate the integral in terms of u.
4. Plug g(x) back in for u.

Alright, these steps don’t seem so bad, but if you really want to learn this, it helps to practice it out in the form of problems, so let’s find the integral of cos(2x) using this process!

There are a few more facts that we need to know to be able to find this integral, and those are as follows:

• The derivative of 2x is 2.
• The integral of cos(x) is sin(x) + C, where C is a constant.
• If a is a constant, then ∫ a ⋅ f(x) dx = a∫ f(x) dx.

Okay, let’s get to work!

First, we notice that if we let f(x) = cos(x) and g(x) = 2x, then f(g(x)) = cos(2x), so we are finding the integral of f(g(x)). Recognizing this makes our substitution much easier, which brings us to our first step, and that is to let u = g(x) and du = g ‘ (x) dx.

Alright, we have that u = 2x and that dx = (1/2)du. The next step is to plug these values into our integral. That is, we plug in u for 2x, and we plug in (1/2)du for dx.

Moving on to the third step, we see that we want to find ∫ (1/2)cos(u) du. To do this, we make use of our facts that we mentioned earlier and find:

We have that ∫ (1/2)cos(u) du = (1/2)sin(u) + C, where C is a constant. We’re almost there! Just one last step! We need to plug 2x back in for u to get (1/2)sin(2x) + C, where C is a constant.

Solution

The integral of cos(2x) is (1/2)sin(2x) + C, where C is a constant.

Checking Your Work

Well, we’ve got our answer, but there were a few steps involved, so we probably want to make sure we didn’t make any mistakes along the way. In other words, it would be nice to be able to check that our answer is correct. Thankfully, we can use derivatives to do so!