Question

solve this question please

Answer 1

$sn \: = n \div 2(2a + (n - 1) \times d)$
where
a=2
d=2
Sn= 210
$210 = n \div 2(2 \times 2 + (n - 1) \times 2)$
$210 = n \div 2(4 + 2n - 2)$
$210 = n \div 2(2n - 2)$
taking out 2 as common we get
$210 = {n}^{2} - n$
${n }^{2} - n - 210$
${n}^{2} + 14n - 15n - 210$
on solving you get
n=15 , -14
n=15