11. $x^2 = (2000)(2007)(2008)(2015)$
ให้ a=2000
$x^2 = a(a+7)(a+8)(a+15)+784$
$x^2 = ([a^2 + 15a] +56)([a^2 + 15a])+784$
$x^2 = (a^2 +15a)^2 +56(a^2 +15a)+784$
$x^2 = (a^2 + 15a+28)^2$
$x= a^2 +15a +28$
$x=(2000)^2 + 15(2000) +28 = 4000000+30000+28 = 4030028$
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