Question 570721
When the equation has the form {{{ f(x) = ax^2 + bx + c }}},
The quadratic formula to find the roots is
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
For real solutions, {{{ b^2 - 4a*c }}} must be {{{ 0 }}} or positive
given:
{{{ 2x^2 - K*x + 8 = 0 }}}
{{{ b^2 - 4a*c = (-K)^2 - 4*2*8 }}}
{{{ b^2 - 4a*c = K^2 - 64 }}}
To be positive or zero,
{{{ K^2 - 64 >= 0 }}}
{{{ K^2 >= 64 }}}
{{{ K >= 8 }}}
and, also, using (-) root
{{{ K <= -8 }}}
so,
{{{ 8 <= K <= -8 }}}