Consider a sphere with the same radius (r = 6) as the cone shown.
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Domain, on an "even root" context, means, an even root cannot have a negative radicand, since say for example 
so... you end up with an "imaginary value"
so... for the case of 20-6x
"x", the domain, or INPUT
can afford to have any value, so long it doesn't make the radicand negative
to check for that, let us make the expression to 0, and see what is "x" then
now, if "x" is 10/3, let's see
now, 0 is not negative, so the radicand is golden
BUT, if "x" has a value higher than 10/3, the radicand turns negative
for example
so.. that's not a good value for "x"
thus, the domain, or values "x" can safely take on, are all real numbers from 10/3 onwards, or to infinity if you wish
so... you end up with an "imaginary value"
so... for the case of 20-6x
"x", the domain, or INPUT
can afford to have any value, so long it doesn't make the radicand negative
to check for that, let us make the expression to 0, and see what is "x" then
now, if "x" is 10/3, let's see
now, 0 is not negative, so the radicand is golden
BUT, if "x" has a value higher than 10/3, the radicand turns negative
for example
so.. that's not a good value for "x"
thus, the domain, or values "x" can safely take on, are all real numbers from 10/3 onwards, or to infinity if you wish