[กิตติ]

21.$(\sqrt[3]{x})^3+(\sqrt[3]{20-x} )^3=20$
$(\sqrt[3]{x}+\sqrt[3]{20-x}) )(\sqrt[3]{x^2}-\sqrt[3]{x^2(20-x)^2} +\sqrt[3]{(20-x)^2} )=20$
$(\sqrt[3]{x^2}-\sqrt[3]{x(20-x)} +\sqrt[3]{(20-x)^2} )=10$.......(1)
$(\sqrt[3]{x}+\sqrt[3]{20-x})^2=4$
$\sqrt[3]{x^2}+2\sqrt[3]{x(20-x)} +\sqrt[3]{(20-x)^2} =4 $..........(2)
(2)-(1)
$3\sqrt[3]{x(20-x)}=-6$
$\sqrt[3]{x(20-x)}=-2$
$x(20-x)+8=0$
$x^2-20x-8=0$
$(x-10)^2-108=0$
รากคำตอบเป็นจำนวนจริง ตอบ $20$