求1,3,8,16,27,41.的通项公式

1＝1
3＝1+2=1+(3-1)
8＝1+2+5=1+(3-1)+(3*2-1)
16＝1+2+5+8=1+(3-1)+(3*2-1)+(3*3-1)
27＝1+2+5+8+11=1+(3-1)+(3*2-1)+(3*3-1)+(3*4-1)
41＝1+2+5+8+11+14=1+(3-1)+(3*2-1)+(3*3-1)+(3*4-1)+(3*5-1)
……
An=1+(3-1)+(3*2-1)+……+[3*(n-1)-1]
=(3n^2-5n+4)/2