Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

## #1 2013-12-09 18:17:38

steveyi
Novice

Offline

### Need help on these 4 TRIG problems

Actually just one question

3. If sin(θ)=−0.6878 and π≤θ≤3π/2, approximate the following to four decimal places.

(a) cos(θ) =  (Round to four decimal places.)
(b) tan(θ) =  (Round to four decimal places.)

It's looking for an answer to (a) and (b). Im completely confused on this problem , dont know what to do.

Last edited by steveyi (2013-12-09 19:04:28)

## #2 2013-12-09 19:43:12

bob bundy
Moderator

Offline

### Re: Need help on these 4 TRIG problems

hi steveyi

Welcome to the forum.

There are formulas connecting sin, cos and tan:

but, as you've got these decimal forms that means you'd have to use a calculator anyway so why not

(i) use a calc. to get theta

(ii) Your calc. will give a negative value so reverse the sign, then cos theta in the range 0 to pi/2

(iii) As tan is positive in the third the value your calc. gives will be the one.

http://www.mathsisfun.com/algebra/trig- … rants.html

Because trig functions are multi-valued and a calculator will only give one answer, you'll always have to do a bit of this to get the values in the right quadrant.

Bob

You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

## #3 2013-12-10 02:55:43

bobbym

Offline

### Re: Need help on these 4 TRIG problems

Hi;

Using the most famous trig identity known ( already given to you in post #1 ) you can get one answer right away.

round to 4 digits .7259

tan(x) you can do.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #4 2013-12-10 04:16:02

steveyi
Novice

Offline

### Re: Need help on these 4 TRIG problems

#### bobbym wrote:

Hi;

Using the most famous trig identity known ( already given to you in post #1 ) you can get one answer right away.

round to 4 digits .7259

tan(x) you can do.

## #5 2013-12-10 04:20:49

anonimnystefy
Real Member

Offline

### Re: Need help on these 4 TRIG problems

Try the negative answer, i.e., -0.725900241...

The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

## #6 2013-12-10 04:26:26

bobbym

Offline

### Re: Need help on these 4 TRIG problems

Hi;

Yes, because of  π ≤ θ ≤ 3π/2.

From the graph you should try the negative of my answer.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

bob bundy
Moderator

Offline