Let set a and set B be set n of natural numbers, and map f: a → B maps element n in set a to element 2n + N in set B, then under map f, the original image of image 20 is______ ．

Let n = 0, 1, 2, 3, 4 So the answer is: 4

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1. Proof: (1 + A + A ^ 2 +...) +a^n）^2-a^n=(1+a+a^2+… +a^n-1)(1+a+a^2+… +A ^ n + 1), where is n natural number
2. Multiply by the power of the base, what if there is a number before the base For example: 2 (x + y) ^ 3 × (x + y) =? I know how to calculate the front one without 2, but I don't know if I add 2 Ask the prawns to help Please give the specific value
3. Under what conditions should we try to add exponentially with the same base
4. Exponents are the same, multiplication or addition? Add or multiply
5. -The reciprocal of the opposite number of 1 and 1 / 3 is? The absolute value is 2 and 1 / 3? The ratio size ↓ 1 (- 4) 3-3-4 power
6. In the polynomial of X obtained by (3x + 32) & nbsp; 100 expansion, the terms with rational coefficients are______ Item
7. How to prove that all polynomials with rational coefficients are countable sets?
8. It is proved that the polynomial f (x) = x ^ 3 + 3x + 1 is irreducible in the field of rational numbers College Advanced Algebra for help!
9. If a polynomial with integral coefficients is reducible in the field of rational numbers, then the polynomial must have a rational root?
10. Write me some laws of polynomials
11. Let a and B be set n of natural numbers, Mapping f: A to B maps element n in set a to element 2 ^ n + N in set B, then under mapping f, the original image of image 11 is
12. It is proved that the cardinality of rational number set in interval (0,1) is equal to that of natural number set!
13. The proof of the equality of cardinal number between algebraic number set and natural number set Don't prove that the set of rational numbers is countable there Let's not just say that algebraic numbers are countable because equations are countable
14. What are natural number sets, rational number sets, real number sets, and how many number sets are there
15. What are natural number set, rational number set and real number set?
16. There is a natural number with two digits. Three times the sum of the two digits is exactly the natural number. Find the natural number
17. 70 is divided into the sum of 11 different natural numbers. How many methods are there? List them one by one
18. How to construct the one to one correspondence between rational number and natural number set
19. The elements of set a are natural numbers If x ∈ a, then 4-x ∈ a 1. Write a set a with only one element 2. Write all sets a with 2 elements 3. How many sets a satisfy the condition of question setting
20. Is n a set of natural numbers or a set of positive integers?