Use a bracket (sometimes called a square bracket) to indicate that the endpoint is included in the interval, a parenthesis (sometimes called a round bracket) to indicate that it is not.
Brackets are like inequalities that say or equal
parentheses are like strict inequalities.
(3,7)includes3.1and3.007and3., but it does not include3. It also includes numbers greater than3and less than7, but it does not include 7.
People sometimes say this is3to7exclusive (Excluding the endpoints)
[4, 9]includes4and every number from4up to9, and it also includes9
People sometimes say this is4to9inclusive (Including the endpoints)
[a,b] = x : a= x and x= b
Of course, mixed intervals(a,b]or[a,b)are also possible.
The symbols-oo(andoo) are used to indicate that there is no left (right) endpoint for the interval. They are not endpoints, but indicators that there is no endpoint. They always take parentheses.
Domain off(x)=sqrtxis[0, oo) (sqrt0 = 0is a number.)
Domain ofg(x)=1/sqrtxis(0, oo) (1/sqrt0is a not number.)
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