Why can't I understand the first math class in senior one? It's too difficult, especially in the first class. I don't understand, and I don't understand my homework. . . . . . . .

Write down the name of the symbol set and hand it in.

When x is an element of set a, it can be recorded as x∈A, and it can be read that x belongs to set a.

. Universal symbol set:

N: non-negative integer set or natural number set {0, 1, 2, 3, ...}

N* or N+: positive integer set {1, 2,3, ...}

Z: integer set {-2,-1, 0, 1, ...}

P: a group of prime numbers

Q: Rational number set

Q+: Set of Positive Rational Numbers

Q-: set of negative rational numbers

R: real number set

R+: positive real number set

R-: negative real number set

C: complex set

φ: an empty set (a set without any elements is called an empty set)

U: complete set (including all the sets discussed in a problem)