The counterexample is () A. x=4B. x=3C. x=2D. x=15

When ∵ x = 3, (x-3) 2 = 0, the counterexample of proving that proposition "x is a real number, then (x-3) 2 > 0" is a false proposition is: x = 3

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1. Proof problem: known: as shown in the figure, in the quadrilateral ABCD, ab ‖ CD, ab = CD. Proof: ad = CB
2. Suppose that vector a + radical 3 vector B is perpendicular to 4 vector A-3 radical 3 vector B, 2 vector a + radical 3 vector B is perpendicular to vector a-radical 3 vector B, And the vectors a and B are not equal to 0, find the angle between a and B
3. The values of cos0 °, tan0 ° and sin0 ° are obtained
4. Let a and B be square matrices of order n, and | a | is not equal to 0. It is proved that AB is similar to ba
5. Let a and B be invertible matrices of order n. how can we prove that the determinant of AB is equal to that of Ba?
6. A. B is a square matrix of order n, and the determinant of a is not zero. It is proved that AB is similar to ba Linear algebra problem
7. It is known that the eigenvalues of a matrix of order 3 are 2,1, - 1. Find the eigenvalues of a + 3E and calculate the determinant | a + 3E|
8. A is a real symmetric matrix of third order, a ^ 2 + 2A = 0, R (a) = 2. Find all eigenvalues of a and the value of determinant | a ^ 2 + 3E | Why R (a) = 2, then - 2 is a double root?
9. Let a and B be m * N and N * m matrices respectively. It is proved that AB and Ba have the same nonzero eigenvalues
10. Let all the elements of the fourth-order matrix a be 1, then the nonzero eigenvalue of a is 1
11. If x and y are all real numbers, and a = xsquare - 2Y + 1, B = ysquare - 2x + 2, it is proved that at least one of a and B is greater than 0
12. Take e, F, G and h on AB, BC, CD and Da of space quadrilateral ABCD If it can intersect with EF and GH at a point P, what is the position of point P?
13. It is known that e, F, G and H are respectively the middle points of AB, BC, CD and DA on the four sides of the spatial quadrilateral ABCD It is known that e, F, G and H are respectively the middle points of AB, BC, CD and DA on the four sides of the space quadrilateral ABCD 1. If BD = 2, AC = 6, then eg & sup2; + HF & sup2= 2. If the angle between AC and BD is 30 °, AC = 6, BD = 4, calculate the area of efgh
14. It is known that the quadrilateral ABCD is a spatial quadrilateral, and e.f.g.h is a spatial quadrilateral AB.BC.CD . da What is the shape of a quadrilateral efgh 2 when AC = BD, the shape of efgh is 3 when AC ⊥ BD, the shape of the quadrilateral efgh is 4 when AC and BD satisfy, the quadrilateral efgh is a square
15. If PAB / PA = 1 / 2, PC / PD = 1 / 3, then the value of FBC / AD is? This is the answer to the 14 questions filled in the blanks in 2010 The right process Wrong number There's no F
16. A quadrilateral ABCD inscribed in a circle, CE ∥ BD intersecting the extension of AB in e AD.BE=BC .DC
17. In quadrilateral ABCD, ad ∥ BC, in order to judge whether quadrilateral ABCD is parallelogram, it should also satisfy () A.∠A+∠C=180° B.∠B+∠D=180° C.∠A+∠B=180° D.∠A+∠D=180°
18. In the parallelogram ABCD, ab = 4, ad = 3, triangle bad = 60, find the area of ABCD
19. M is the midpoint of edge ad of parallelogram ABCD, and MB = MC. Can you explain that parallelogram ABCD must be a rectangle ten minutes
20. In the parallelogram ABCD, ad = 2ad, M is the midpoint of AB, connecting DM and MC. What is the positional relationship between DM and MC? Please prove AB=2AD Revision of the previous question