Bài giải
a) Ta có: A = 550 - 548 + 546 - 544 +...+ 56 - 54 + 52 - 1
=> A = (550 - 548) + (546 - 544) +...+ (56 - 54) + (52 - 1)
=> A = (548.52 - 548.1) + (544.52 - 544.1) +...+
(54.52 - 54.1) + 50.(52 - 1)
=> A = 548.(52 - 1) + 544.(52 - 1) +...+ 54.(52 - 1) +
50.(52 - 1)
=> A = (52 - 1).(548 + 544 +...+ 54 + 50)