A.3=1.2.3+2.3.3+.....+98.99.3
A.3=1.2.(0+3)+2.3.(4-1)+.....+98.99.(100-97)
A.3=0.1.2+1.2.3+2.3.4-1.2.3+.....+97.98.99-98.99.100
A.3=0.1.2+(1.2.3-1.2.3)+(2.3.4-2.3.4)+.....+(97.98.99-97.98.99)+98.99.100
A.3=0.1.2+98.99.100=0+98.99.100
A=98.99.100/3=323400
A = 1.2 + 2.3 +...+ 98.99
3A = 1.2.3 + 2.3.(4-1) +...+ 98.99.(100-97)
3A-A = 1.2.3 +2.3.(4-1) - 1.2.3 +...+ 98.99.(100-97) - 97.98.(99-96)
A = 98.99.100: 3
A = 323 400
B=1.(101-2)+2.(101-3)+3.(101-4)+.....+99.(101-100)
B=101.1-2.1+101.2-3.2+.....+101.98-99.98+101.99-100.99
B=(101.1+101.2+101.3+......+101.99)-(1.2+2.3+....+99.100)
B=101.(1+2+3+....+99)-(1.2+2.3+....+99.100)
B=101.4950-(1.2+2.3+....+99.100)
B=499950-(1.2+2.3+....+99.100)(làm tiếp tương tự câu A)
B=499950-333300=166650
A=1.2 +2.3 +..... + 98.99
3A= 1.2.3 + 2.3.3 + 3.4.3 +....... +98.99.3
3A=1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + .... + 98.99(100 - 97)
3A=1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ..... + 98.99.100 - 97.98.99
=>\(\dfrac{98.99.100}{3}\)
