Question

if the last term of an ap=2n + 1 then find sum of the first three terms

Answer 1

hope this helps you to get the answer

Answer 2

ap = 2n + 1

a = 2×1+1 = 3
a2 = 2×2+1=5
a3 = 2×3+1 = 7

so a = 3
d = 2

sn =
$\frac{n}{2} (2a + (n - 1)d) \\ \frac{3}{2} (2 \times 3 + (3 - 1)2) \\ \frac{3}{2} (6 + 4) \\ \frac{3}{2} \times 10 \\ 3 \times 5 \\ 15$
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