Đặt A = 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + 9 + ... + 301 - 302 - 303 + 304 + 305
=> A = 1 + ( 2 - 3 - 4 + 5 ) + ( 6 - 7 - 8 + 9 ) + .... + ( 301 - 302 - 303 + 304 ) + 305
=> A = 1 + 0 + 0 + ... + 0 + 0 + 305
=> A = 306
1+2-3-4+..................+300-301-302+303+304
=1+(2-3-4+5)+..............+(300-301-302+303)+304
=1+0+......................+0+304
=1+304
=305
306 nha ban hoi nay mik lam lon
k chomik nha ^_^ ^_^ ^_^
1+2-3-4+5+6-7-8+9+....+296-297-298+299+300-301-302+304+305
=[1+(2-3-4+5)+(6-7-8+9)+...+(296-297-298+299)+(300-301-302+304)]+305
=[1+ 0 + 0 +...+ 0 + (-1) ]+305
=0+305
=305