When x takes what value, 2x + 1 is negative even number? 3x-1 is positive odd number?

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• 1. Odd + even = () odd + odd = () odd even = () even odd = () odd x odd = () odd x even =（ Odd + even = () odd + odd = () Odd even = () even odd = () Odd x odd = () odd x even = ()
• 2. Odd + odd = (), odd odd = () odd + even = (), odd x even = ()
• 3. X is an even number, 3x is an odd number, this sentence is right or wrong? Fast, who quickly adopt who
• 4. If n is used to represent any natural number, then even numbers can be represented by 2n, and odd numbers can be represented by 2n______ Express
• 5. In the 100 natural numbers 1-100, the sum of all even numbers is more than the sum of all odd numbers______ ．
• 6. A is a non-zero natural number A、1/a÷3＜1/3÷a B、1/a÷3＝1/3÷a C、1/a÷3＞1/3÷a
• 7. If a is a non-zero natural number, then a ＞ B ＜ C = D should be filled in 0 of a / 18 ＞ A / 25
• 8. A is a non-zero natural number. The largest result in the following formula is (). ① a × 0.1 ② a △ 0.1 ③ a + 0.1
• 9. a. B is a non-zero natural number, which satisfies the following condition (1) 1 / 7 at the same time
• 10. There are seven different natural numbers, and the difference between at least two numbers is a multiple of six
• 11. In natural numbers less than 20, odd numbers containing prime numbers and composite numbers have; in natural numbers less than 20, even numbers containing prime numbers and composite numbers have
• 12. Natural number is divided into odd number and even number according to (). It can also be divided into 1, prime number and composite number according to ()
• 13. Let n be a natural number, then three consecutive odd numbers can be expressed as (), three consecutive even numbers can be expressed as (), and three consecutive integers can be expressed as () It's urgent. It will be handed in soon
• 14. Use n to represent any natural number, then you can use () to represent even number and () to represent odd number
• 15. How to express any natural number n, odd number, even number?
• 16. Let n be a natural number, then the odd number can be expressed as (), and the even number can be expressed as (), (why)
• 17. How many even numbers are there in natural numbers 1 to 100? How many odd numbers are there?
• 18. Add a plus or minus sign randomly between 1-2011 consecutive natural numbers (but the result should be positive), is the result odd or even? Be detailed!
• 19. In the 100 natural numbers 1-100, the sum of all even numbers is more than the sum of all odd numbers______ ．
• 20. How many even numbers are there in natural number 1 ~ 100, and how many odd numbers are there