answer:Since "x=2" implies "(x+1)(x-2)=0", and "(x+1)(x-2)=0" implies "x=2 or x=-1", therefore, "x=2" is a sufficient but not necessary condition for "(x+1)(x-2)=0". Hence, the correct choice is boxed{A}.

answer:Let's analyze each proposition individually:① For complex numbers a+bi and c+di to be equal, we must have a+bi = c+di. This equality holds if and only if a=c and b=d. Proposition ① is therefore correct.② Saying that the magnitudes of any complex number cannot be compared is incorrect. While it is true that there is no natural ordering for complex numbers, real numbers (a subset of complex numbers) can indeed be compared. Thus, proposition ② is incorrect.③ If overrightarrow {z_{1}} = overrightarrow {z_{2}}, then by definition of vector equality, every component of overrightarrow {z_{1}} must be equal to the corresponding component of overrightarrow {z_{2}}. This implies that the magnitudes, or the modulus, of those complex numbers are equal. Therefore, the statement | overrightarrow {z_{1}} | = | overrightarrow {z_{2}} | is correct, making proposition ③ correct.④ If | overrightarrow {z_{1}} | = | overrightarrow {z_{2}} |, this implies that the two complex numbers have the same magnitude. However, this condition alone does not guarantee that the vectors themselves are equal or negatives of each other. There are infinitely many complex numbers with the same magnitude but different arguments (i.e., they are not co-linear). Hence, proposition ④ is incorrect.In conclusion, there are two incorrect propositions, ② and ④.The correct answer is boxed{B}.

answer:First, we calculate z=(2-i)^2. According to the rules of complex numbers, we have:z=(2-i)^2=2^2-2times2i+i^2=4-4i+(-1)=3-4i.Next, we find the modulus of z. The modulus of a complex number a+bi is given by sqrt{a^2+b^2}. Therefore, the modulus of z=3-4i is:|z|=sqrt{3^2+(-4)^2}=sqrt{9+16}=sqrt{25}=5.So, the answer is: boxed{5}.To solve this problem, we used the rules of complex number arithmetic to simplify the given expression. Then, we applied the formula for the modulus of a complex number to find the required value. This problem primarily tests the ability to calculate the modulus of a complex number and to simplify complex expressions using the rules of complex number arithmetic.

answer:In one cubic kilometer - a billion cubic meters.## SolutionIn one cubic kilometer, there is a billion cubic meters (1000 in length, 1000 in width, and 1000 in height). If all of them are laid out in a row, it would form a strip one billion meters long, i.e., one million kilometers.## Answer1000000 km.

answer:# Answer: 20Solution. Let's calculate how many times the side of the original square is repeated in the sum of all perimeters. The sides of the rectangle are counted once (a total of 12), and the crossbars are counted twice (4 cdot 2 + 10 cdot 2 = 28). In total, 40.800: 40=20 cm - the side of the square.

answer:Since a > b > c > 0,We have 2ab > 2ac > 2bc,Now, let's simplify x, y, and z:x = sqrt {a^{2}+(b+c)^{2}} = sqrt {a^{2}+b^{2}+c^{2}+2bc},y = sqrt {b^{2}+(c+a)^{2}} = sqrt {a^{2}+b^{2}+c^{2}+2ac},z = sqrt {c^{2}+(a+b)^{2}} = sqrt {a^{2}+b^{2}+c^{2}+2ab},Hence, z > y > x.So, the answer is: boxed{z > y > x}.This is because a > b > c > 0 implies 2ab > 2ac > 2bc. Substituting these values into the expressions for x, y, and z gives us the result.This problem tests understanding of the properties of inequalities and the application of the multiplication formula, requiring both reasoning and computational skills. It is a basic-level problem.