89+899+8999+89999+899999
=90-1+900-1+...+900000-1
=90+900+...+900000-1*5
=999999-5
=999994
解:
=(90-1)+(900-1)+(9000-1)+(90000-1)+(900000-1)
=999990-1-1-1-1-1
=999990-(1+1+1+1+1)
=999985
89+899+8999+89999+899999
=90+900+9000+90000+900000-5
=999990-5
=999985
90+900+9000+90000+900000-1-1-1-1-1
=999990-5
=999985