Friday

January 30, 2015

January 30, 2015

Posted by **Kate** on Friday, January 29, 2010 at 4:25pm.

find the exact value do not use a calculator

cot[(-5pi)/12]

Here is my attempt

RT = square root

pi = 3.14...

cot[(-5pi)/12]={tan[(-5pi)/12]}^-1={tan[(2pi)/12-(8pi)/12]}^-1={tan[pi/6-(2pi)/3]}^-1={(tan[pi/6]-tan[(2pi)/3])/(1+tan[pi/6]tan[(2pi)/3])}^-1={((RT[3])/3+RT[3])/(1+((RT[3])/3)(-RT3))}^-1

then I got this

(RT[3] + 3RT[3])/3

over

1 - 3/3

as you can see this is a problem

because it leaves me with a denomenator of 0

- PreCalculus -
**Damon**, Friday, January 29, 2010 at 4:43pmcot (-5pi/12)

well that will be in quadrant 4 since it is clockwise from the x axis due to the minus sign and 5 pi/12 is less than pi/2 or 6 pi/12.

call the x component of this vector a and the y component b. Then the magnitude of the cotan is a/b.

Notice that the angle to the negative y axis is 6pi/12 - 5pi/12 = pi/12

then the magnitude of tan pi/12 is a/b, just what we are looking for

so what is tan pi/12?

In degrees it is 15 degrees.

We know tan 30 degrees = 1/sqrt3 and sin 30 =1/2 and cos 30 = sqrt3 /2

so what is tan 15 degrees?

well

tan 15 = sin 30/(1+cos 30)

I think you can take it from there.

- PreCalculus -
**Damon**, Friday, January 29, 2010 at 4:48pmBy the way remember it is in quadrant 4 so negative. I just used sizes in that 30, 60, 90 triangle and ignored signs.

**Answer this Question**

**Related Questions**

MATH! HELP!!! - I am putting < for the square root abbreviation. I don't know...

Precalculus - What is the square root of 2 to the poweer of (1/3)? In order to ...

Precalculus - Find cot x if sin x cot x csc x = .square root 2 After reducing i ...

College Algebra - I need some help with a handful of questions. I need to see ...

College Algebra - I need some help with a handful of questions. I need to see ...

College Algebra help please - I need some help with a handful of questions. I ...

Algebra - Use the quadratic formula to solve the equation. Give exact answers: ...

Precalculus - PLEASE HELP ME WITH THESE QUESTION AS MUCH AS POSSIBLE!! Evaluate...

trigonometry - Use fundamental Identities to find the exact value of the ...

precalculus - find all solutions of each equations a) cos2x=square root of 3/2 b...