Oplossing
$
\eqalign{
& \sqrt {x - 4} - \sqrt {21 - x} + \sqrt {4x - 11} = 0 \cr
& \sqrt {x - 4} + \sqrt {4x - 11} = \sqrt {21 - x} \cr
& \left( {\sqrt {x - 4} + \sqrt {4x - 11} } \right)^2 = \left( {\sqrt {21 - x} } \right)^2 \cr
& x - 4 + 2\sqrt {x - 4} \sqrt {4x - 11} + 4x - 11 = 21 - x \cr
& 2\sqrt {x - 4} \sqrt {4x - 11} = 36 - 6x \cr
& \sqrt {x - 4} \sqrt {4x - 11} = 18 - 3x \cr
& \left( {\sqrt {x - 4} \sqrt {4x - 11} } \right)^2 = \left( {18 - 3x} \right)^2 \cr
& (x - 4)(4x - 11) = 9x^2 - 108x + 324 \cr
& 4x^2 - 27x + 44 = 9x^2 - 108x + 324 \cr
& 5x^2 - 81x + 280 = 0 \cr
& (x - 5)(5x - 56) = 0 \cr
& x = 5 \vee x = 11\frac{1}
{5}\,\,(v.n.) \cr
& x = 5 \cr}
$
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