What does the mathematical expression 9 ∉ T mean? Algebra Expressions, Equations, and Functions Variable Expressions 1 Answer sente · Room 204 Dec 14, 2015 #9 !in T# means that #9# is not an element of the set #T#. Explanation: The symbol #!in# is used with a set #S# and a potential element #a# of #S# to mean #a !in S# if and only if #a# is not an element of #S#. Some people like to say that #a# "does not belong" to set #T#. (compare to #in# which implies an element is in a set) Thus #9 !in T# means that #9# is not an element of the set #T# Answer link Related questions How do you write the variable expression for: a quotient of 2 and the sum of a number and 3 ? What are variables? What are variable expressions? How do you write variable expressions? How do you evaluate variable expressions? How do you simplify the expression #3x-x+4#? How do you write a quotient of a number and 6 as an expression? How do you evaluate the expression #2x+1# for #x=1#? How do you write a product of a number and 2 as an expression? How do you write 5 less than 2 times a number as a variable expression? See all questions in Variable Expressions Impact of this question 5873 views around the world You can reuse this answer Creative Commons License