# Need Trigonometry help

## Homework Statement

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

## 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
Homework Helper
Try writing sin65 as sin(25+40).

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

ehild

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
Staff Emeritus