a1=3,a2=3.3,a3=3.9,a4=4,8
an=a(n-1)+(n-1)×0.3
a(n-1)=a(n-2)+(n-2)×0.3
...
a2=a1+0.3
累加得
an=a1+(1+2+3+...+n-1)×0.3
=3+n(n-1)×1/2×0.3
=3n²/20-3n/20+3