In the arithmetic sequence {an}, if a1 + A2 + a3 = - 24, A18 + A19 + A20 = 78, then S20= a18-a1=17d a19-a2=17d a20-a3=17d So (A18 + A19 + A20) - (a1 + A2A + 3) = 3 * 17D = 102 D=2 a1+a2+a3=3a2=-24 a2=-8=a1+d a1=-10 A20 = a1 + 19d = 28 S20=(a1+a20)*20/2=180 ]

In the arithmetic sequence {an}, if a1 + A2 + a3 = - 24, A18 + A19 + A20 = 78, then S20= a18-a1=17d a19-a2=17d a20-a3=17d So (A18 + A19 + A20) - (a1 + A2A + 3) = 3 * 17D = 102 D=2 a1+a2+a3=3a2=-24 a2=-8=a1+d a1=-10 A20 = a1 + 19d = 28 S20=(a1+a20)*20/2=180 ]

Arithmetic sequence an = a1 + (n-1) d
A20 = 20a1 does not hold
First of all, the value of D is wrong, d = 1, so A1 = 7
Then I'll give you the value directly, S20 = 330 ask: a1 + A2 + a3 = 3a2 = - 24
a2=-8=a1+d
2A1 = A2, right? Then A2 = - 16, isn't m + n = j + K in the arithmetic sequence, can am + an = AJ + AK
A construction site uses 34 tons of cement in September, of which 89 tons are used in the second half of the month?
According to the meaning of the question: the output ratio of the first half of the month and the second half of the month is 9:8; then the consumption in the first half of the month is 34 (9 + 8) × 9, = 34 (17) × 9, = 18 (tons); in the second half of the month: 34-18 = 16 (tons); a: 18 tons are used in the first half of the month, and 16 tons are used in the second half of the month
There are 42 people in a tour group, 6 less men than women. How many men and women are there in the tour group?
(2) The area of the new square is 120% more than that of the old square. If the sum of the area of the new square and the old square is 30000 square meters, the area of the new square will increase
(3) How many guest rooms are there? How many people are there?
(1) Suppose there are x men and Y women
x+y=42
x=2y-6
x=26
y=16
(2) The new square covers an area of X 10000 square meters and the old square covers an area of Y 10000 square meters
x=（1+120%）y
x+y=3
x=2.0625
y=0.9375
(3) There are x rooms and y people
y=7x+6
y=8x-1
X=7
y=55
If S20 = 100 and a1 + A2 + a3 = 4, then A18 + A19 + A20 = ()
A. 20B. 24C. 26D. 30
In the arithmetic sequence, ∵ a1 + A2 + a3 = 3a2 = 4, ∵ A2 = 43, ∵ S20 = 100, ∵ S20 = 20 (a1 + A19) 2 = 10 (a1 + A19) = 10 (A2 + A19) = 100, ∵ A2 + A19 = 10, ∵ A19 = 10-a2 = 10-43 = 263,