# Domain of a radical function | Functions and their graphs | Algebra II | Khan Academy

Find the domain

of f of x is equal to the principal square

root of 2x minus 8. So the domain of

a function is just the set of all of the possible

valid inputs into the function, or all of the possible

values for which the function is defined. And when we look at how the

function is defined, right over here, as the square root,

the principal square root of 2x minus 8, it’s only

going to be defined when it’s taking the

principal square root of a non-negative number. And so 2x minus

8, it’s only going to be defined when 2x minus 8

is greater than or equal to 0. It can be 0, because then you

just take the square root of 0 is 0. It can be positive. But if this was negative,

then all of a sudden, this principle square root

function, which we’re assuming is just the plain vanilla

one for real numbers, it would not be defined. So this function definition is

only defined when 2x minus 8 is greater than or equal to 0. And then we could

say if 2x minus 8 has to be greater

than or equal to 0, we can solve this

inequality to see what it’s saying about

what x has to be. So if we add 8 to both

sides of this inequality, you get– so let me just

add 8 to both sides. These 8’s cancel out. You get 2x is greater

than or equal to 8. 0 plus 8 is 8. And then you divide

both sides by 2. Since 2 is a

positive number, you don’t have to swap

the inequality. So you divide both sides by 2. And you get x needs to be

greater than or equal to 4. So the domain here is the

set of all real numbers that are greater than

or equal to 4. x has to be greater

than or equal to 4. Or another way of saying

it is this function is defined when x is

greater than or equal to 4. And we’re done.

Thanks Sal

Second! !!

so for these types of problems do you only use greater than or equal to? never less than or equal to?

@mystickybuns1 In this case, greater than or equal to is used because x can equal four, or anything greater than four. If x=4, than we would be taking the square root of zero, which is zero. Less than or equal to cannot be used here because x cannot equal anything less than four, as this would cause a negative number under the square root, giving us an imaginary number as an answer. To answer your question, the equation dictates whether or not to use greater than or equal to or less than/equalto

"plain vanilla principle root". I appreciate so much that you explain everything in clear, everyday language that non-math majors can understand! Plus a little humor thrown in! You are so educated, so knowledgeable, but you dont speak like a pompous know-it-all. Your explanations are life-savers for students who are otherwise drowning in "problem-based learning" & inadequate, poorly worded instruction at their schools. THANK YOU!!

How come Sal says that the domain is x >= 4. Shouldn't it also be x belongs to real numbers? Also, would the range be y >= 0, because the minimum value is zero? For a function, do we express the domain and range in terms of x and y, or do we just state them. Also, when is it that we have to put the domain and range in the curly brackets?

God bless you Sir

still confused as to why it is greater than or equal to…