# Identifying Triogonometric Identities - ?

1. Jul 3, 2011

### nukeman

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

I am having issues with these. Does anyone know of a tutorial for these, like something I can print out that has sample problems with solutions on how to solve them?

Thanks very much!!

(example problem question)

(sinx + cosx)^2 = sin 2x + 1

I am just having a hard time with these, so any info on a good tutorial/worksheet online would be great!

2. Relevant equations

3. The attempt at a solution

2. Jul 3, 2011

### eumyang

Have you checked these sites? (Admittedly, I haven't look at the trig stuff, but other topics.)
http://www.purplemath.com/modules/index.htm" [Broken] (look at the bottom for trig tutorials)
http://www.khanacademy.org/#trigonometry"

Regarding your example:
$$(\sin x + \cos x)^2 = \sin 2x + 1$$

Let me ask you, how would you start this problem?

Last edited by a moderator: May 5, 2017
3. Jul 3, 2011

### Mentallic

There's a reason why we have 2. Relevant equations

4. Jul 3, 2011

### nukeman

Ok great, thanks ill take a look at those!

To answer your question:

My first step is usually my problem. I don't understand how you would know your first step, or line of attack for solving a question like this.

Last edited by a moderator: May 5, 2017
5. Jul 3, 2011

### Mentallic

Well you're trying to show they equal each other, so you can go about it two ways. Either work with the left hand side and make it turn into the right, or work with the right and turn it into the left. Since the left is a perfect square, and you know how to expand, that's always the best approach.

6. Jul 3, 2011

### nukeman

Right, so usually I would start with the left side because its usually the more complicated side.

So take my above question. What am I looking to do to the left side. Yes I know I must make/prove it = to the right side, but what am I looking to do first?

7. Jul 4, 2011

### eumyang

Did you read this from Mentallic's post?
He gave you the first step, so try it.

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