Find real roots $x_1$, $x_2$ of equation $x^5-55x+21=0$, if we know $x_1\cdot x_2=1$
1. Given the equation \(x^5 - 55x + 21 = 0\) and the condition \(x_1 \cdot x_2 = 1\), we need to find the real roots \(x_1\) and \(x_2\).

2. The given polynomial can be factored as \((x^2 - 3x + 1)(x^3 + 3x^2 + 8x + 21) = 0\).

3. First, we solve the quadratic factor \(x^2 - 3x + 1 = 0\):
   \[
   x = \frac{3 \pm \sqrt{9 - 4}}{2} = \frac{3 \pm \sqrt{5}}{2}
   \]
   Therefore, the roots of the quadratic factor are:
   \[
   x_1 = \frac{3 + \sqrt{5}}{2}, \quad x_2 = \frac{3 - \sqrt{5}}{2}
   \]
   These roots satisfy the condition \(x_1 \cdot x_2 = 1\) because:
   \[
   \left(\frac{3 + \sqrt{5}}{2}\right) \left(\frac{3 - \sqrt{5}}{2}\right) = \frac{(3 + \sqrt{5})(3 - \sqrt{5})}{4} = \frac{9 - 5}{4} = 1
   \]

4. Next, we consider the cubic factor \(x^3 + 3x^2 + 8x + 21 = 0\). To find its roots, we use the substitution \(y = x + 1\), transforming the equation into:
   \[
   (y - 1)^3 + 3(y - 1)^2 + 8(y - 1) + 21 = 0
   \]
   Simplifying, we get:
   \[
   y^3 - 3y^2 + 3y - 1 + 3y^2 - 6y + 3 + 8y - 8 + 21 = 0
   \]
   \[
   y^3 + 5y + 15 = 0
   \]

5. To solve \(y^3 + 5y + 15 = 0\), we use the method of Cardano's formula. Let \(y = u + v\), then:
   \[
   u^3 + v^3 + (u+v)(u^2 - uv + v^2) + 5(u+v) + 15 = 0
   \]
   Setting \(u^3 + v^3 = -15\) and \(uv = -\frac{5}{3}\), we solve the quadratic in \(u^3\) and \(v^3\):
   \[
   t^2 + 15t - \left(\frac{5}{3}\right)^3 = 0
   \]
   \[
   t^2 + 15t - \frac{125}{27} = 0
   \]
   Solving this quadratic equation:
   \[
   t = \frac{-15 \pm \sqrt{225 + \frac{500}{27}}}{2} = \frac{-15 \pm \sqrt{\frac{6075 + 500}{27}}}{2} = \frac{-15 \pm \sqrt{\frac{6575}{27}}}{2}
   \]
   \[
   t = \frac{-15 \pm \frac{\sqrt{6575}}{\sqrt{27}}}{2} = \frac{-15 \pm \frac{\sqrt{243 \cdot 27}}{3\sqrt{3}}}{2} = \frac{-15 \pm \frac{9\sqrt{3}}{3}}{2} = \frac{-15 \pm 3\sqrt{3}}{2}
   \]

6. The roots of the cubic equation are:
   \[
   y = -1 + \frac{\sqrt[3]{-1620 + 60\sqrt{789}} - \sqrt[3]{1620 + 60\sqrt{789}}}{6}
   \]
   and two complex roots:
   \[
   y = -1 + \frac{\sqrt[3]{1620 + 60\sqrt{789}} - \sqrt[3]{-1620 + 60\sqrt{789}}}{12} \pm \frac{\sqrt{3}\left(\sqrt[3]{1620 + 60\sqrt{789}} + \sqrt[3]{-1620 + 60\sqrt{789}}\right)}{12}i
   \]

Conclusion:
The real roots of the equation \(x^5 - 55x + 21 = 0\) are:
\[
x_1 = \frac{3 + \sqrt{5}}{2}, \quad x_2 = \frac{3 - \sqrt{5}}{2}
\]

The final answer is \(\boxed{\left\{\frac{3 + \sqrt{5}}{2}, \frac{3 - \sqrt{5}}{2}\right\}}\)