[질문] 등비수열합구하는공식알려주세요

등비수열의합Sn을구하는공식적용하기1)r<1일때Sn=a×(1-rn)/(1-r)를r>1일때는Sn=a×(rn-1)/(r-1)를적용2)끝항이주어진경우,arn=r×arn-1=rl이므로Sn=(rl-a)/(r-1)^^