6.等差数列{an}\{a_{n}\}{an}的前nnn项和为SnS_{n}Sn,a1=15a_{1}=15a1=15,a2+a3+a4+a5=30a_{2}+a_{3}+a_{4}+a_{5}=30a2+a3+a4+a5=30,则SnS_{n}Sn的最大值为((( )))