1. Write the following series respectively.

(1) A sequence of prime numbers in descending order from 0 to 20.

Prime number, also known as prime number, means that among all integers greater than 1, there is no other factor (divisor) except 1 and itself. This integer is called a prime number.

Prime numbers between 0 and 20 are in descending order: 2, 3, 5, 7, 1 1, 13, 17, 19.

(2) The positive square root sequence of composite numbers in descending order from 0 to 20.

A composite number is a positive integer divisible by other positive integers except 1 and itself, and 2 is not a composite number.

The positive square root of the composite number between 0 and 20 is in descending order:

The square roots of radical numbers 4(2) and 6 are 8, 9(3), 10, 12, 14, 15, 16(4) and 18.

2. Observe the characteristics of the following series, fill in the blanks with appropriate numbers and write the general formula of each series.

( 1) ( 1 ), -4, 9, (- 16 ),25,(-36 ),49

The general formula is: (-1) to the power of (n+ 1) times the power of n.

(2) 1, radical number 2, (radical number 3), 2, radical number 5, (square root of 6), radical number 7)

A general formula is: root number n