a, 55 - x + 33 = 76
=> - x = 76 - 55 - 33
=> - x = - 12
=> x = 12
vậy_
b, (x + 1) + (x + 2) + (x + 3) +.......+ (x + 100) = 5550
=> x + 1 + x + 2 + x + 3 + ... + x + 100 = 5550
=> (x + x + x + ... + x) + (1 + 2 + 3 + ... + 100) = 5550
=> 100x + 5050 = 5550
=> 100x = 500
=> x = 5
vậy_