<s>[INST] What is the minimum polynomial of $x = \sqrt{2}+\sqrt{3}+\sqrt{4}+\sqrt{6} = \cot (7.5^\circ)$? Inspired by a previous question what let  $x = \sqrt{2}+\sqrt{3}+\sqrt{4}+\sqrt{6} = \cot (7.5^\circ)$.  What is the minimal polynomial of $x$ ?
The theory of algebraic extensions says the degree is $4$ since we have the degree of the field extension $[\mathbb{Q}(\sqrt{2}, \sqrt{3}, \sqrt{4}, \sqrt{6}): \mathbb{Q}] = [\mathbb{Q}(\sqrt{2}, \sqrt{3}): \mathbb{Q}] =4$
Does trigonometry help us find the other three conjugate roots?


*

*$+\sqrt{2}-\sqrt{3}+\sqrt{4}-\sqrt{6} = \cot \theta_1$

*$-\sqrt{2}+\sqrt{3}+\sqrt{4}-\sqrt{6} = \cot \theta_2$

*$-\sqrt{2}-\sqrt{3}+\sqrt{4}+\sqrt{6} = \cot \theta_3$


This problem would be easier if we used $\cos$  instead of $\cot$.  If I remember the half-angle identity or... double-angle identity:
$$ \cot \theta = \frac{\cos \theta}{\sin \theta} = \sqrt{\frac{1 - \sin \frac{\theta}{2}}{1 + \sin \frac{\theta}{2}}}$$
Sorry I am forgetting, but I am asking about the relationship between trigonometry and the Galois theory of this number.
 [/INST] All you need to do is write your number and its square, cube and fourth powers in the form $z^j = a_j + b_j \sqrt{2} + c_j \sqrt{3} + d_j \sqrt{6}$ with $a_j,b_j,c_j,d_j$ rational and solve a system of four equations in four unknowns 
$$ 
\eqalign{a_4 + a_3 x_3 + a_2 x_2 + a_1 x_1 + a_0 x_0  &= 0\cr
b_4 + b_3 x_3 + b_2 x_2 + b_1 x_1 + b_0 x_0  &= 0\cr
c_4 + c_3 x_3 + c_2 x_2 + c_1 x_1 + c_0 x_0  &= 0\cr
d_4 + d_3 x_3 + d_2 x_2 + d_1 x_1 + d_0 x_0  &= 0\cr
}$$
to determine a polynomial $Z^4 + x_3 Z^3 + x_2 Z^2 + x_1 Z + x_0$ of which your number is a root. 
Maple says the answer is $Z^4 - 8 Z^3 + 2 Z^2 + 8 Z + 1$.
EDIT: Here is another way, maybe a bit deeper.  Your number is $z = 1+(1+\sqrt{2})(1+\sqrt{3})$.
$u = 1+\sqrt{2}$ is an eigenvalue of $A = \pmatrix{0 & 1\cr 1 & 2\cr}$, and
$v = 1+\sqrt{3}$ is an eigenvalue of $B = \pmatrix{0 & 2\cr 1 & 2\cr}$.
So $uv = (1+\sqrt{2})(1+\sqrt{3})$ is an eigenvalue of 
$$ A \otimes B = \pmatrix{0 & 0 & 0 & 2\cr 0 & 0 & 1 & 2\cr 0 & 2 & 0 & 4\cr 1 & 2 & 2 & 4}$$
which has characteristic polynomial $X^4 - 4 X^3 - 16 X^2 - 8 X + 4$.  Substitute $X = Z - 1$ to obtain $Z^4 - 8 Z^3 + 2 Z^2 + 8 Z + 1$. 
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