c) C= $\frac{4x}{\sqrt{x}-2}$ (x>0; $x\neq4; x\neq9)$
$C^2=40C ⇔$\(\left[ \begin{array}{l}C=0\\C=40\end{array} \right.\)
TH1: C=0 ⇔x=0 (loại)
TH2: C=40 ⇔ $\frac{4x}{\sqrt{x}-2}=40⇔x=10(\sqrt{x}-2)⇔x-10\sqrt{x}+20=0$
⇔\(\left[ \begin{array}{l}\sqrt{x}=5-\sqrt{5}\\\sqrt{x}=5+\sqrt{5}\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=30-10\sqrt{5}\\x=30+10\sqrt{5}\end{array} \right.\)