84+85+86+87+88+89+90
=87-3+87-2+87-1+87+87+1+87+2+87+3
=87*7
=609
215+217+219+221+223
=219-4+219-2+219+219+2+219+4
=219*5
=（220-1）*5
=1100-5
=1095