Trigonometry Help: Solving Sin and Cos Equations - Homework Problem

[rain.]

Homework Statement

I can't find way to solve $$\frac{Sin25 * Sin65}{Cos 40}$$

Homework Equations

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.

 

[rock.freak667]

Try writing sin65 as sin(25+40).

 

[Mentallic]

Try writing sin65 as sin(25+40).

I don't see how that would help.

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

Which means what i tried above doesn't work.

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

 

[rock.freak667]

I don't see how that would help.

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

 

[Mentallic]

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

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.

 

[ehild]

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

ehild

 

[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).

 

[vela]

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.

 

[Mentallic]

I also need good reference for line (Ax+By+C=0) and circle ((x - a) ^2 + (y - b)^2 = r^2).

Sorry what do you mean by this?