# Need Trigonometry help

1. Aug 19, 2010

### rain.

1. The problem statement, all variables and given/known data
I can't find way to solve $$\frac{Sin25 * Sin65}{Cos 40}$$

2. Relevant equations

3. The attempt at a solution
I looked trough allformulas i have I came up with:
$$\frac{Sin25 * Sin65}{Cos 40} = \frac{Sin25 * Cos25}{Cos 40}$$

But then found that formula is:
$$Sin 2a =2{Sina * Cosa}$$

Which means what i tried above doesn't work.

2. Aug 19, 2010

### rock.freak667

Try writing sin65 as sin(25+40).

3. Aug 19, 2010

### Mentallic

I don't see how that would help.

Why not? You gave up on this idea too quickly. Just one extra step and everything will be blatantly obvious.

4. Aug 19, 2010

### rock.freak667

I wasn't too sure if it would, I was hoping the cos40 would cancel on expansion.

5. Aug 19, 2010

### Mentallic

Yeah I know where you were coming from I actually thought it would work too so I quickly scribbled some stuff down but it didn't seem to go anywhere far.

6. Aug 19, 2010

### ehild

Some more hint: 40=65-25; 90=65+25

ehild

7. Aug 19, 2010

### rain.

thanx.

Is there any goog tuts for learning trigonometry on my own? My textbook is terrible, it starts with hard things soon as explanations part is over. I need something that begins with easier things (but not too easy). I also need good reference for line (Ax+By+C=0) and circle ((x - a) ^2 + (y - b)^2 = r^2).

8. Aug 19, 2010

### vela

Staff Emeritus
I haven't actually looked through it, but the Schaum's outline for trig is probably a good resource to complement your textbook. Schaum's outlines typically summarize the material and have worked example as well as practice problems with answers.

9. Aug 20, 2010

### Mentallic

Sorry what do you mean by this?