a) 5x + 5x+2 = 650
5x . (1 + 52) = 650
5x . 26 = 650
5x = 650:26 = 25
5x = 52
Vậy x = 2
b) Đặt A= 1*2 + 2*3 + ... + 99*100
3A = 1*2*3 + 2*3*(4-1) + ....... + 99*100*(101-98)
3A = 1*2*3 + 2*3*4 - 1*2*3 + ......99*100*101 - 98*100*101
3A = (1*2*3 - 1*2*3) + ...... + (98*99*100) + 99*100*101
3A = 99*100*101 = 999900
Vậy A = 999900 : 3 = 333300