\require{AMSmath}

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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