Posted by David on .
if cot 2θ = 5/12 with 0≤2θ≤π find cos θ, sin θ, tan θ

Trigonometry 
Reiny,
if cot 2Ø = 5/12
then tan 2Ø = 12/5
recognizing the familar 51213 rightangled triangle, we would say
sin 2Ø/cos 2Ø = 12/5 = (12/13) / (5/13)
so sin 2Ø = 12/13 > 2sinØcosØ = 12/13
and cos 2Ø = 2cos^ Ø 1 = 5/13
2cos^2 Ø = 18/13
cos^2 Ø = 9/13
cosØ = ± 3/√13
if cosØ = 3/√13
in 2sinØcoØ = 12/13
sinØ(3/√13) = 6/13
sinØ = (6/√13)(√13/3) = 2√13/13 or 2/√13
tanØ = 2/3
if cosØ = 3/√13
(only the signs will change, same method)
sinØ = 2/√13
tanØ = 2/3 
Trigonometry 
Steve,
but, of course, cos2θ will not be 3/√13, since cot2θ>0 and 2θ<π

Steve  Trigonometry 
Reiny,
Steve, you are right, should have looked at that original restriction more carefully.
( must have automatically assumed the usual
0 ≤ Ø ≤ 2π )
So you will find my other solution comical, where I assumed that domain and got solutions in all 4 quadrants :
...
If cot(2Ø) = 5/12 , then
tan(2Ø) = 12/5 and 2Ø is in either I or III
tan 2Ø = 2tanØ/(1  tan^2 Ø)
let tanØ = u
then 12/5 = 2u/(1  u^2) , (for easier typing)
10u = 12  12u^2
12u^2 + 10u  12 = 0
6u^2 + 5u  6 = 0
(2u + 3)(3u  2) = 0
u = 3/2 or u = 2/3
tanØ = 3/2 or tan u = 2/3
case 1:
tanØ = 3/2> Ø in II or IV
make a sketch of a right rightangled triangle
hypotenuse = √(3^2+2^2) = √13
IN II: sinØ = 2/√13 , cosØ = 3/√13, tanØ = 3/2
IN IV: sinØ = 2/√13 , cosØ = 3/√13, tanØ = 3/2
case 2:
tanØ = 2/3> Ø in I or III
make a new sketch, hypotenuse is still √13
IN I: sinØ = 2/√13, cosØ = 3/√13 , tanØ = 2/3
IN III: sinØ = 2/√13 , cosØ = 3/√13, tanØ = 2/3