Let the sum of the first n terms of the arithmetic sequence {an} be Sn, A3 = 11, S15 > 0, S16

If S15 = 15 (a1 + A15) / 2 = 15 (A7 + A8) / 2 > 0, a7 + A8 > 0
S16=16(a1+a16)/2=16*2*a8/2=16a80
Therefore, starting from the eighth item, the following items are less than 0
So S7 is the largest

A particle starts to move in a straight line from static state. The acceleration in the first second is 5 m / S2, and the acceleration in the second is - 5 m / S2. Repeat the situation in the first and second seconds in the third and fourth seconds. What is the displacement of the particle when the time is 100 seconds? When the displacement of the particle reaches 56.25m, how much time does the particle pass?

(1) From vt = V0 + at, we can get that the velocity at the end of 1s is: V1 = a1t1 = 5 × 1 = 5 (M / s), the velocity at the end of 2S is: V2 = V1 + a2t2 = 5 + (- 5) × 1 = 0, the particle starts to move in a straight line from rest, the acceleration in 1s is 5m / S2, so the V-T image is an oblique straight line, the velocity at the end of 1s is 5m / s, and the acceleration in 2S is 5m / s

Xiao Ming's father took a day off every four days, but his mother took a day off every five days. On March 4, both father and mother took a rest at home, and then went to work Month They can rest at home at the same time on Sunday

5 and 6 are coprime numbers, so the least common multiple of 5 and 6 is: 5 × 6 = 30, so on March 4, they rest at the same time, so the next time they rest at the same time is: 4 + 30 = 34, that is, April 3; so the answer is: 4, 3

Mathematical problems of first degree equation of one variable in elementary school
When solving the equation 3 (x + 5) + 5 [(x + 5) - 1] = 7 (x + 5) - 1, one student first removed the brackets and got 3x + 15 + 5x + 25-5 = 7x + 35-1, then moved the term and merged the similar terms. Do you have a simpler solution?

Take x + 5 as a whole. Don't remove the brackets
3(X+5)+5[(X+5)-1]=7(X+5)-1
3(X+5)+5(X+5）-5=7(X+5)-1
（3+5-7）（X+5）=-1+5
X+5=-1+5
x=-1

Given that a is a root of the equation x & # 178; + 2x-1 = 0 about X, find the value of 3A & # 178; + 6A + 2013

The answer is 2016. According to the meaning of the question, if a = x, then x ^ 2 + 2x = 1, that is, a ^ 2 + 2A = 1, and 3a ^ 2 + 6A + 2013 = 3 (a ^ 2 + 2a) + 2013 = 2016

A math problem in grade one of junior high school
The known equations are 1. 2x + y = 5m + 6
2. x-2y=-17
If the solutions of 1 and 2 are negative, then M =?

-9

If the tolerance D of the arithmetic sequence (an) is not equal to zero, and A1, A3 and A9 are equal proportion sequence, then (A2 + A4 + A8) divided by (a1 + a3 + A9) =?

14/13
∵ A1, A3, A9 are equal ratio series
∴a1*(a1+8d)=(a1+2d)^2
4a1*d=4d^2
a1=d
(a2+a4+a8)/(a1+a3+a9)=(3a1+11d)/(3a1+10d)=14/13

3500 cubic decimeters = () cubic meters 205 cubic meters = () square decimeters 4200 square centimeters = () square decimeters 2.25 liters = () ml = () cubic centimeters

3500 cubic decimeter = (3.5) cubic meter
205 cubic meters = () square decimeters
4200 square centimeter = (42) square decimeter
2.25 L = (2250) ml = (2250) CC

After the Yushu earthquake in Qinghai Province, a company has to transport a batch of relief materials to the disaster area. If it is transported by a truck of our factory, it will take 10 times
complete
After the Yushu earthquake in Qinghai Province, a company has to transport a batch of relief materials to the disaster area. If it is transported by a truck of our factory, it will take 10 times; if it is transported by a large truck of our transportation company, it will only take 5 times. Now, after four times of transportation by a truck of our factory, the rest is transported by a truck of our factory and a large truck of our transportation company How many times will it take to finish the next shipment?

You left this question too! Here's what I did: (the teacher is right) set the rest to y times
One tenth by four plus one fifth by y plus one tenth by y = 1
The final result was twice

I want to make 20 cubic meters of concrete floor, using pc32.5 cement, how many tons of cement? How many cubic meters of river sand? How many cubic meters of stone sand?

It depends on what strength you are going to make. I'll give you the following list: C20 mix proportion 1:2.45:4.12:0.65 cement 6000kg sand 14680kg stone 24720kg water 3900kgc25 mix proportion 1:2.40:3.60:0.65 cement 6400kg sand 15360kg stone 23060kg water 4160kgc30 mix proportion 1:1.95:3.05

Let the sum of the first n terms of the arithmetic sequence {an} be Sn, A3 = 11, S15 > 0, S16