Question

How to know sum of positive terms for 85,78,71….1.

Answer 1

$a=first \: term$

$d=common \: difference$

$here,a=85,d=-7$

$tn=a+(n-1)d$

$tn=85+(n-1)-7$

$tn=85+(7n+7)$

$92-7n=0$

$7n=92$

$n=92/7$

$n=13.14$

$n=13$

$Sn=(n/2)(2a+(n-1)d)$

$sn=13/2(2×85+(13-1)(-7)$

$sn=(170+(12×-7)$

$sn=170-84$

$sn=13/2×86$

$sn=559$