Question

Determine a soma dos múltiplos de 3 compreendidos entre 80 e 1000?

Answer 1

a1=81
razão=3
an=999
n=?

an=a1+(n-1).r
999=81+(n-1).3
999=81+3n-3
999-81+3=3n
921=3n
n=921/3
n=307

sn=(a1+an).n/2
s307=(81+999).307/2
s307=1080.307/2
s307=331560/2
s307=165780