设数列an=a*n^3+b*n^2+c*n+d
a1=a+b+c+d=4
a2=8a+4b+2c+d=11
a3=27a+9b+3c+d=36
a4=64a+16b+4c+d=67
a=-2
b=21
c=-42
d=27
an=-2n^3+21n^2-42n+27
a5=92