# Find the sum of the first 25 terms of an arithmetic sequence whose first term is -9 and whose common difference is 5.?

### 5 Answers

- la consoleLv 71 year ago
For an arithmetic sequence:

a₁ = - 9

a₂ = a₁ + d ← where d is the common difference, i.e.: 5

a₃ = a₂ + d = a₁ + 2d

a₃ = a₃ + d = a₁ + 3d

…and you can generalize writing:

a(n) = a₁ + (n - 1).d → for the 25th term, n = 25

a₂₅ = - 9 + (25 - 1).5

a₂₅ = - 9 + 120

a₂₅ = 111

s = a₁ + a₂ + a₃ + a₃ + … + a₂₅

s = (- 9) + (- 4) + (1) + (6) + … + (111)

s = (111) + (106) + (101) + (96) + … + (- 9)

2s = (102) * 25

s = 51 * 25

s = 1275

- 1 year ago
T[1] = -9 = -14 + 5 * 1

T[n] = -14 + 5 * n

T[25] = -14 + 5 * 25 = 125 - 14 = 111

S[n] = (n/2) * (T[1] + T[n])

S[25] = (25/2) * (-9 + 111)

S[25] = (25/2) * (102)

S[25] = 25 * 51

S[25] = 25 * 50 + 25 * 1

S[25] = 1250 + 25

S[25] = 1275

- LearnerLv 71 year ago
Apply the formula

Sum to n terms of an AP =

(n/2)*[2a + (n - 1)d]

where n = number of terms = 25

a is first term = -9

d is common difference = 5

Ans: 1275

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