A group of soldiers advance at a constant speed on the road at V1 = 2m / s, with a team length of 3200m. The correspondent sends a command from the tail of the team to the head of the team by motorcycle at V2 = 18m / s. after arriving at the head of the team, he immediately turns around and returns to the tail of the team at the same speed. How long does it take for the correspondent to go back and forth once? The two trains run in opposite directions, and the speed is 10m / s and 15m / s respectively. It takes 6S for the passenger on the first train to see the second train passing him. If the two trains run in the same direction, when the second train exceeds the first train, how long does it take for the passenger to see the second train passing him?

A: First, to the end: T1 = s / v = 3200 / (18-2) = 200s
And then to the end: T2 = s / v = 3200 / (18 + 2) = 160S
Add up: 200s + 160S = 360s = 6min
B:S2=vs=(10+15)*6s=150m
t=s/v=150/(15-10)=30s

The circumference of a square is 36cm. If the length is reduced by 4cm and the width is increased by 2cm, the rectangle will become a square. Find the side length of the square

Let the side length of the square be x, according to the meaning 2 * [(x + 4) + (X-2)] = 36. X = 10

Simple calculation 3.8 * 98 + 7.6
54*(210-195)/30
fast

3.8*98+7.6
=3.8*98+3.8*2
=3.8*（98+2）
=3.8*100
=380
54*（210-195）/30
=54*（210-200+5）/30
=54*15/30
=27

The function f (x) is an odd function defined on (- 1,1). When x belongs to (0,1), f (x) = 2 ^ 3-x. find the range of function f (x)

Firstly, the domain of definition is divided into (0,1) (- 1,0) and x = 0. On (0,1), when x = 0, we can calculate f (x) = 8; when x = 1, we can calculate f (x) = 4; and f (x) itself is monotone, so in (0,1), the range of value is (4,8). Because f (x) is an odd function, so f (x) + F (- x) = 0, so f (x) + F (- x) = 0

A square with a side length of 2 cm and equal perimeter and area______ (judge right or wrong)

The perimeter is 2 × 4 = 8 (CM); the area is 2 × 2 = 4 (cm 2); the unit of perimeter and area is different, so the size cannot be compared

35,35,35.; 33,32; 1,3,5,7,9; 2,4,6,8,10; 1,4

35, 35, 35
33,32 -- three by two
1, 3, 5, 7, 9 - everything
2, 4, 6, 8, 10 -- no coincidence
1, 4 - one after another

It is known that the sum of the first n terms of the sequence {an} is Sn, A1 = 1, and 2nsn + 1-2 (n + 1) Sn = n & # 178; + n (n ∈ n *)
The general term formula of the sequence {an}

2nSn+1－2(n+1)Sn=n²+n
2n (n + 1) on both sides
∴ S(n+1)/(n+1) - Sn/n=1/2
{Sn / N} is an arithmetic sequence, the first term is A1 / 1 = 1, and the tolerance is 1 / 2
∴ Sn/n=1+(1/2)(n-1)=(n+1)/2
∴ Sn=n(n+1)/2
(1) n=1,a1=1
(2) n≥2
an=Sn-S(n-1)
=n(n+1)/2-n(n-1)/2
=n*[(n+1)-(n-1)]/2
=n
N = 1 also satisfies the above equation
∴ an=n

Decorate a sitting room, with the side length of 2 decimeters square brick floor, need 500 pieces, with the side length of 4 decimeters square brick floor, need several pieces（

Hehe, this is simple, first calculate the general layout of the room: 2 * 2 * 500 = 20000 square decimeters;
Divide by the floor area of each brick to get the number of bricks needed: 20000 / (4 * 4) = 125

16 vs 14 0.25 vs 0.4 two thirds vs two fifths 1 vs four fifths 0.02 vs 0.48 one quarter vs 0.25 4 vs one quarter 0.3 vs 9

16:14 = 8:7
4 = 5:8
Two thirds vs two fifths = 5:3
1:4 = 5:4
48 = 1:24
Quarter ratio 0.25 = 1:1
4:1 = 16:1
3 to 9 = 1:30

Lim x tends to the third power of 3, x-3 / x-27,

LIM (x-3) (X & # 178; + 3x + 9) / (x-3) x tends to 3
=LIM (X & # 178; + 3x + 9) x tends to 3
=27