Question

If sum of first n term of an ap is 2 and square + 5 and then find S20

Answer 1

$s = 2 {n}^{2} + 5 \\ s| |1| | = a |1| = 7 \\ s |2| = 13 \\ s |2| = a |1| + a |2| = 7 + a |2| = 13 \\ a |2 | = 6$
common difference =second term-first term
d=6-7=-1
s(20)=2(20*20)+5=805