It is known that {an} is an arithmetic sequence. If a1 + A5 + A9 = π, then the value of COS (A2 + A8) is () A. −12B. −32C. 12D. 32

It is known that {an} is an arithmetic sequence. If a1 + A5 + A9 = π, then the value of COS (A2 + A8) is () A. −12B. −32C. 12D. 32

Because {an} is an arithmetic sequence, and a1 + A5 + A9 = π, according to the property of arithmetic sequence, there is A5 = π 3, so A2 + A8 = 2 π 3, so cos (A2 + A8) = - 12, so choose & nbsp; a

In the arithmetic sequence {an}, S4 = 1, S8 = 4, then what is the value of A17 + A18 + A19 + A20?
If you encounter a similar problem next time, what's a good solution?

Let the first item be A1 and the tolerance be d
From Sn = A1 * n + n * (n-1) / 2,
S4=4a1+6d=1,
S8=8a1+28d=4,
Solution
a1=1/16,
d=1/8.
a17+a18+a19+a20=S20-S16=4a1+70d=9
☆⌒_ - [hope to help you~

In the arithmetic sequence {an}, we know that A5 = 3.8, a8 = 5.9 A1=

A8-A5=2.1
D-2.1/3=0.7
A1=A5-4*D=1

Let {an} be the sum of the first n terms of the arithmetic sequence and Sn, if S4 > = 10, S5

∵ S4≥10 ∴ 4(a1+a4)/2≥10 ∴a1+a4≥5 ∴2a1+3d≥5
∵ S5≤15 ∴ 5(a1+a5)/2≤15 ∴a1+a5≤6 ∴ a1+2d≤3
∵ -(2a1+3d)≤-5
∴ a4=a1+d=-(2a1+3d)+3(a1+2d)≤-5+9=4
The maximum value of A4 is 4

A car moves at a speed of 20 m / s on a horizontal highway. From a certain moment, after closing the throttle, the car decelerates evenly. The acceleration is 0.5 m / S2. Find: (1) the total displacement of the car decelerates; (2) the displacement within 5 s before the car stops

(1) According to the velocity displacement formula of uniform speed change motion, the total displacement of the car is s = V2 − v202a = 0 − 2022 × (− 0.5) = 400m; (2) the car does uniform deceleration linear motion, the final speed is zero, and the reverse process is uniform acceleration linear motion with zero initial speed, and the displacement 5 seconds before the car stops: s' = 12at2 = 12 × 0.5 × 52 = 6.25m; (1) the total displacement of the car deceleration motion is 400m (2) the displacement is 6.25m within 5 seconds before the vehicle stops moving

Xiaogang and Xiaoqiang have 128 stamps in total. Xiaoqiang has three times as many stamps as Xiaogang. How many stamps does Xiaogang have

128 / (3 + 1) = 32

First grade history exercises
In ancient times, there were many diseases among the people. There was a work by sages, drilling flint to make fire, so as to melt (remove) the fishy smell. But the people said (were happy), so that the king (elected him as the leader) would be called suirenshi. What's the reason of "many diseases among the people"?

Because before Suiren's fire, people can only eat raw food, nutrition is not easy to absorb and easy to cause disease

A point representing an integer on the number axis becomes an integral point. The unit length of a certain number axis is one centimeter. If you draw a line ab of 50 cm randomly on the number axis
Then the number of integral points covered by line AB is? And the reason

When the end point of line segment is on the whole point, there are 51 whole points covered by ab; when the end point of line segment is not on the whole point, there are 50 whole points covered by ab

According to research, the temperature T (℃) at H (m) above the ground has the following relationship with the surface temperature T (℃): T = t-kh, the temperature at 150 (m) above the ground is 8.8 ℃, and the temperature at 400 (m) above the ground is 6.8 ℃. Please estimate that the temperature at 2500 (m) above the ground is (℃)
A. -10℃B. -15℃C. -20℃D. -25℃

According to the meaning of the question, when h = 150, t = 8.8, that is, 8.8 = t-150k; when h = 400, t = 6.8, that is, 6.8 = t-400k; simultaneous equations can be solved, t = 10, k = 1125; the analytical formula can be t = 10-1125h; substituting H = 2500, t = - 10

Given x = ^ m + n = 3, y = ^ m + N + 2, find the value of the algebraic formula (1 / 3x ^ my ^ n) * (- 1 / 2x ^ NY ^ m)

It should be x ^ (M + n) = 3, y ^ (M + n) = 2,
（1/3x^my^n)*（-1/2x^ny^m)
=-1/6*x^（m+n）*y^（m+n）
=-1/6*3*2
=-1.