Question

Awarding 90 Points

The 12th term of an arithmetic series is 62.
The 20th term of the series is 102.
Find the sum of the first 20 terms of this arithmetic series.

Answer 1

• 1090

Explanation:

Given AP

• a₁₂ = 62
• a₂₀ = 102

To find

• S₂₀

Solution

• a₁₂ = a + 11d = 62
• a₂₀ = a + 19d = 102

Subtract the first equation from the second one:

• 8d = 40
• d = 5

Find a:

• a + 11*5 = 62
• a = 62 - 55
• a = 7

Find the sum of the first 20 terms:

• S₂₀ = 1/2*20(a + a₂₀) = 10(7 + 102) = 10(109)= 1090