Solving an equation in quaternions How to solve equation $x^4=1$ in quaternions?
I know how to solve that equation in complex numbers but I have no idea how to do it in quaternions.
I have also a question about general way to solve equations in quaternions.  And is there similar theorem to fundamental algebra theorem which connects degree of polynomial equation with number of quaternion roots of that equation?
EDIT:
My task was to find roots $x^2+1=0$
EDIT2:
How to prove that $x^2+1=0$ has infinite number of roots?
I represented x as $a+bi+cj+dk$ and get equations: $a^2−b^2−c^2−d^2=−1$ and $ab+cd=0$, $ac−bd=0$, $ad+cb=0$. Is it worth to solve that or is there a smarter way?

Let me consolidate my two comments into an answer that may be a bit clearer to the OP.
You want to find all the roots of the polynomial $x^4 - 1$ in the quaternions $\Bbb{H}$. Note that $x^4 - 1 = (x^2 - 1)(x^2 + 1)$, just as in $\Bbb{R}$ or $\Bbb{C}$ and note that $\Bbb{H}$ has no non-trivial zero divisors: if $X, Y \in \Bbb{H}$, then $XY = 0$ implies $X = 0$ or $Y = 0$. So $X^4 -1 = 0$ iff either $X^2 - 1 = 0$ or $X^2 + 1 = 0$.
For a general element $X = a + bi + cj+ dk$ of $\Bbb{H}$, where $a, b, c, d \in \Bbb{R}$, we have the identity:
$$
X^2 = (a + bi + cj+ dk)^2 = a^2 - b^2 - c^2 - d^2 + 2a(bi + cj + dk)
$$
which you can verify by multiplying out using the formulas $ij = k$, $jk = i$, $ji = -k$ etc.
Thus, comparing cooefficients, $X^ 2 - 1 = 0$ iff:
$$
a^2 - b^2 - c^2 - d^2 = 1 \\
ab = ac = ad = 0
$$
From the first of those equations, we must have $a \neq 0$, but then from the other equations we must have $b = c = d = 0$, implying that $a^2= 1$. So the only solutions to $x^2 - 1 = 0$ are the quaternions $X = \pm 1$.
Likewise, if $X^2 + 1 = 0$, we must have:
$$
a^2 - b^2 - c^2 - d^2 = -1 \\
ab = ac = ad = 0
$$
And then the first equation tells us that at least one of $b$, $c$ or $d$ is non-zero, from which one of the the other equations will tell us that $a = 0$. So the solutions of $x^2 + 1 = 0$, are the quaternions $X = bi + cj + dk$ where $b^2 + c^2 + d^2 = 1$.
Putting that all together gives us all the solutions of $x^4 + 1 = 0$: they are the quaternions $X$ such that $X = \pm 1$ or $X = bi + cj + dk$ where $b^2 + c^2 + d^2 = 1$.
