If cosa = 2,1, what is cos2a

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• 1. In the survey, the slope of the slope is & # 190;, let α be the slope angle, then what is cos α equal to?
• 2. Is cos 110 ° equal to 0? Is cos 90 ° equal to 0?
• 3. Given that cos θ is equal to minus 3 / 5, and θ belongs to (π, 3 / 2 π), find the value of θ of COS 2
• 4. What is cos θ / 2
• 5. If cos α = 1 / 3 and α belongs to (π, 2 π), then cos π / 2 is equal to
• 6. If a / b equals cos A / cos B, then what triangle is it
• 7. What is two thirds of cos
• 8. Sin angle = 3 / 5, angle in (Pie / 2, pie), find cos angle equal to
• 9. What degree is cos α equal to 0.8
• 10. If cos θ = - 1213, θ∈ (π, 3 π 2), then the value of COS (θ + π 4) is equal to______ ．
• 11. A question about "an empty set is a subset of an empty set" If we judge by "any set is a subset of itself", an empty set is a subset of an empty set. That is to say, an empty set is an element of an empty set According to the definition of an empty set, a set without any elements is called an empty set?
• 12. Why is an empty set a subset of any set? Shouldn't the concept of subset be included first? So any set contains an empty set? Why? What's the difference between inclusion and belonging? Is it because there will always be blank space left in Venn chart?
• 13. Questions about proving that "Φ is a subset of any set" Most of the books use the counter proof method (assuming that the existing element x belongs to Φ, but X does not belong to set a, then "Φ is not a subset of any set". But because there is no element in Φ, the previous assumption that "Φ is not a subset of any set" is wrong, that is, the conclusion that "Φ is a subset of any set" is correct) But I don't think this proof is right. Another way of thinking, if you want to prove that "Φ is a subset of any set", as long as you can prove that (element x belongs to Φ, and X belongs to set a), but because there is no element in Φ, it is wrong to assume that (element x belongs to Φ, and X belongs to set a), that is, "Φ is not a subset of any set". Is it true that "Φ is not a subset of any set"? Therefore, I think it is not feasible to prove that "Φ is a subset of any set" by using the two "tools" of the definition of Φ (there is no element in Φ) and the definition of subset (if the element x belongs to a and X belongs to B, then a is a subset of B), Therefore, I think that "Φ is a subset of any set" should be an artificial definition, rather than a theorem deduced by reverse proof Definition of subsets [if (all) elements x belong to set a, then x belongs to set B if and only if a is a subset of B] Let proposition p: (all) elements x belong to set a; Q: X belongs to set B; Z: A is a subset of B; The definition of subsets can be transformed into propositional formula: ZP - > Q (1) Let a: the element x belongs to Φ; b: The element x belongs to any set a; c: Φ is a subset of any set a; It is impossible to bring in the above proposition formula (1) to prove that it is a permanent truth formula, that is, proposition a = t, B = f, because the truth value of proposition a is always = t (the definition of Φ), so proposition C is always = t, that is, "Φ is a subset of any set a"... Proof over! Is that right?
• 14. It is known that the set a = {12345} B = {678} (1) maps from a subset of B to a, of which one-to-one mapping is Known set a = {12345} B = {678} (1) Mapping from a subset of B to a, one by one mapping is (2) In the mapping from a to B, there is exactly one element in B In the mapping from a to B, one element in B has no original image
• 15. Let u = {1,2,3,4,5}, a and B be subsets of u, if a ∩ B = {1}, a ∩ Cu, B = {2}, CUA ∩ Cu B = {3,5}, find the set a, B,
• 16. To select two different subsets from the subset of set u = {a, B, C, D}, the following two conditions should be satisfied: (1) both a and B should be selected; (2) For any two selected subsets A and B, a must belong to or equal to B or B must belong to or equal to A. how many different selection methods are there? Please list them all
• 17. Set a = {an isosceles triangle with 2 sides and 30 angles}, then the number of subsets of set a is? And the answer is 16
• 18. The number of all subsets of set {3.5} is___________ individual
• 19. The number of subsets of set {a, B, C} containing element a is
• 20. How many proper subsets of set a = {1,2,3,4,5,6,7,8}