Question

Please express in interval notation:

x < 0 or 7< x < 9

Answer 1

x < 0 or 7< x < 9

Split into two parts:
x < 0
(-∞,0)
i.e. from negative infinity to 0, excluding both limits using the parentheses.

7< x < 9
(7,9)
i.e. from 7 to 9, again excluding the two limits.
Now put them together with the union notation, ∪
(-∞,0) ∪ (7,9)

Answer 2

There is an alternative form for the interval notation for excluding the limits, i.e. instead of using parenthese, we could use brackets pointing to the exterior of the values, in the above example, we <i>could</i> write it as:
]-∞,0[ ∪ ]7,9[

Answer 3

(-∞,0) ∪ (7,9)

Answer 4

(-∞,0) ∪ (7,9)
or
]-∞,0[ ∪ ]7,9[