HELP~高二一道求数列前n项和的题

n=2k:
Sn=(-1+4)+(-7+10)+...+[-(3n-5)+(3n-2)]=3k=3n/2
n=2k+1
Sn=(-1+4)+(-7+10)+...+[-(3n-8)+(3n-5)]-(3n-2)=3k-(3n-2)=(-3n+1)/2
Sn=[1-(-1)ⁿ+6n(-1)ⁿ]/4