# CosX in terms of Tanx?

1. Jan 30, 2007

### 06Sport

1. The problem statement, all variables and given/known data
seems simple, but i am stumped. Says write cos(x) in terms of tan(x).

2. Relevant equations
would this be a reciprocal equation? or a Pythagorean? I'm lost

3. The attempt at a solution

i dont even know where to begin.

2. Jan 30, 2007

### mathwonk

sin/cos = tan, so cos= sin/tan. har har.

can you use derivatives?

3. Jan 30, 2007

### Moridin

Write down the two formula for tan x and cos x for a right angle triangle. Are there any similar terms in those equations?

Edit: Beaten to it.

4. Jan 30, 2007

### mathwonk

DO YOU KNOW WHaT TAN' IS? or 1 + tan^2?

5. Jan 30, 2007

### 06Sport

the angle is unknown. I think thats why its confusing me.

sin/cos = tan, so cos= sin/tan - these are what i have. But would that be the answer? tan= sin/cos ? or cos=sin/tan?

6. Jan 30, 2007

### mathwonk

i was joking. read my second post.

7. Jan 30, 2007

### 06Sport

now im even more confused.

would it be cos=sin/tan?

8. Jan 30, 2007

### arildno

How can you express sine in terms of cosine?

9. Jan 30, 2007

### 06Sport

i dont know

10. Jan 30, 2007

### arildno

Well, what RELATION exists between the sine and cosine of an angle?

11. Jan 30, 2007

### @/@

$\sin x= \sqrt{1-cos^2x}$

Last edited: Jan 30, 2007
12. Jan 30, 2007

### 06Sport

hmm, sin/cos=tan, cos/sin=cot, sin^2 + cos^2=1

i need cos(theta) in terms of tan(theta) though. Unless thats what we are working up to :)

13. Jan 30, 2007

### arildno

So, look at the last identity you posted.

What do you get by dividing ôn both sides with cos^{2} ?

14. Jan 30, 2007

### 06Sport

sin^2 = 1/cos^2?

15. Jan 30, 2007

### @/@

16. Jan 30, 2007

### arildno

Don't you know how to divide an equation with a number?

17. Jan 30, 2007

### blackcat

you can express sine in terms of cosine as cos (x-90) where x is in degrees or in radians cos (x-pi/2).

18. Jan 30, 2007

### drpizza

Am I the first person who thinks it can't be done? Maybe I'm overlooking something, but I'm seeing a sign problem. (+/- when you solve)

19. Jan 30, 2007

### drpizza

Only works for 1st and 2nd quadrant angles, that is, angles between 0 and 180 degrees. (or between 0 and 2Pi). Plus, it works for 0 degrees and 180 degrees. If you're in the 3rd or 4th quadrant, then you'd have to use a negative square root.

Last edited: Jan 30, 2007
20. Jan 30, 2007

### @/@

yes,
how to use +- in latex?