a5=a1+4d
a6=a2+4d
a7=a3+4d
a8=a4+4d
所以:
a5+a6+a7+a8
=a1+a2+a3+a4+16d
=80+16d
=6480
则d=(6480-80)/16=400
而:
a2=a1+d
a3=a1+2d
a4=a1+3d
得:
a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4a1+6d
所以:
4a1+6*400=80
4a1=80-2400=-2320
a1=580
6480
a5=a1+4d
a6=a2+4d
a7=a3+4d
a8=a4+4d
所以:
a5+a6+a7+a8
=a1+a2+a3+a4+16d
=80+16d
=6480
则d=(6480-80)/16=400
而:
a2=a1+d
a3=a1+2d
a4=a1+3d
得:
a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4a1+6d
所以:
4a1+6*400=80
4a1=80-2400=-2320
a1=580
6480