It is known that △ ABC is an equilateral triangle, EA and CD are perpendicular to plane ABC, e and D are on the same side of plane ABC, and EA = AB = 2A, DC = a, f is the midpoint of be The verification: (1) DF ‖ plane ABC (2) AF ⊥ plane EDB

It is known that △ ABC is an equilateral triangle, EA and CD are perpendicular to plane ABC, e and D are on the same side of plane ABC, and EA = AB = 2A, DC = a, f is the midpoint of be The verification: (1) DF ‖ plane ABC (2) AF ⊥ plane EDB

1) Through point F, do FG ⊥ AB and G, connect CGEA, because FG ⊥ EA is equal to half of EA, so EA ⊥ plane ABC, EA = a = CDEA ∥ CD, so quadrilateral fdcg is parallelogram, so DF ⊥ CG, so DF ⊥ plane ABC 2) from 1, we can see that CG ⊥ AB is CG ⊥ EA, so CG ⊥ EAB, so CG ⊥ AF (1)

Given that the four roots of the equation (x2-2x + m) (x2-2x + n) = 0 form an arithmetic sequence with the first term of 14, then | M-N | is equal to ()
A. 1B. 34C. 12D. 38

Let four roots be x1, X2, X3 and X4 respectively, then X1 + x2 = 2, X3 + X4 = 2. According to the property of arithmetic sequence, when m + n = P + Q, am + an = AP + aq. Let X1 be the first term, and X2 be the fourth term, then the sequence can be 14, 34, 54, 74, M = 716, n = 1516

1. Lim [(1-A) / 2A] ^ n = 0, then the value range of real number a is
2. Let Lim [(an ^ 2 + bn-1) / (4N ^ 2-5n + 1)] = 1 / B, then a * b =?
3. It is known that the sequence an is an arithmetic sequence, the tolerance D ≠ 0, and A1, A2 (1,2 are the angle marks, the same below) are two of the equations x ^ 2-a3 * x + A4 = 0 about X, then an =?
4. It is known that the number of terms of an is even, all items are positive, the sum of all terms is four times of the sum of even terms, and A2 * A4 = 9 (A3 + A4). How many terms of {lgan} have the largest sum?

1.
∵lim[(1-a)/2a]^n=0 ,∴-1