Question

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The 8t h term in the arithmetic sequence is 17, and 12t h term is 25. Find the first
term, and the sum of the first 20 terms.

Answer 1

Step-by-step explanation:

t8 = a1 + (n - 1)*d

t8 = 17

17 = a1 + 7*d

t12 = 25

25 = a1 + 11d

17 = a1 + 7d Subtract

8 = 4d Divide by 4

8/4 = 4d/4

2 = d

17 = a1 + 7d

17 = a1 + 7*2

17 = a1 + 14 Subtract 14

3 = a1

Sum 20 terms

The 20 term = a1 + 19*2

The 20 term = 3 + 38

= 41

Sum = (a1 + a20) * 20 / 2

Sum = (3 + 41)* 20/2

Sum = 44 * 10

Sum = 440