5 Using a formula find the sum of the first 20 terms in the

5.) Using a formula, find the sum of the first 20 terms in the sequence: 5,8 11,14

Solution

Solution

5) To find the sum of the first \'n\' terms of the sequence use the formula,

S_n=n(a₁+a₂) /2

where n is the number of terms, a₁ is the first term and a_n is the last term

the sum of the first 20 terms of the series if a₁=5 and a₂₀ = 62

S₂₀=20(5+62)/2

S₂₀ = 670

6) Given the geometric series is 18,-6,2,-2/3,.....

Then Common Ratio is

Dividing the second term by the first term

-6/18 = -1/3

2/-6 = -1/3

Therefore each entry -1/3 yields the next entry.