Đặt A=1.3.5 + 3.5.7 + 5.7.9 + ................ + 95.97.99 8A= 1.3.5.8 + 3.5.7.8 + 5.7.9.8 + ................ + 95.97.99.88A=1.3.5(7+1)+3.5.7(9-1)+5.7.9.(11-3)+.......+95.97.99(101-93)8A=3.5.7+15+3.5.7.9-3.5.7+5.7.9.11-3.5.7.9+.......+95.97.99.101-93.95.97.998A=15+95.97.99.101 A= A=11517600
Đặt A=1.3.5 + 3.5.7 + 5.7.9 + ................ + 95.97.99 8A= 1.3.5.8 + 3.5.7.8 + 5.7.9.8 + ................ + 95.97.99.88A=1.3.5(7+1)+3.5.7(9-1)+5.7.9.(11-3)+.......+95.97.99(101-93)8A=3.5.7+15+3.5.7.9-3.5.7+5.7.9.11-3.5.7.9+.......+95.97.99.101-93.95.97.998A=15+95.97.99.101 A= A=11517600đúng thì tick v bên trái nha