Question

if the nth term of an AP is 9-5n . Find the sum of the first 5
terms

Answer 1

$a_{1} = 9 - 5 \times 1 \\ = 4 \\ a_{2} = 9 - 5 \times 2 \\ = - 1 \\ d = - 1 -4 \\ = - 5 \\ s_{5} = \frac{5}{2} (2 \times 4 + 4 \times - 5) \\ = \frac{5}{2} \times - 12 \\ = 5 \times - 6 \\ = - 30$