Friday

February 27, 2015

February 27, 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 - Use the exact values of the sin, cos and tan of pi/3 and pi/6, and the ...

calculus - I need help on finding the local linear approximation of tan 62 ...

math - Use the exact values of the sin, cos and tan of pi/3 and pi/6, and the ...

Precalculus/Trig 5 - Find the exact value of thet following trigonometric ...

Math - Could someone explain the steps for this question. Simplify the ...

Trigonometry - Find the exact value of tan(a-b) sin a = 4/5, -3pi/2<a<-pi...

trig - please help me with some questions I skipped on a review for our test ...

math - use exact values of the sin,cos and tan of (pi/3) and (pi/6)(which I have...

Trig - Given: cos u = 3/5; 0 < u < pi/2 cos v = 5/13; 3pi/2 < v < ...

math - use exact values of the sin,cos and tan of (pi/3) and (pi/6)(which I ...