Ta có: 3A=1.2.3+2.3.3+3.4.3+.....+99.100.3
3A=1.2.3+2.3.(4-1)+3.4..(5-2)+....+99.100.(101-98)
3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+....+99.100.101-98.99.100
3A=99.100.101
A=\(\frac{99.100.101}{3}\)
A=333300
Đặt A= 1.2+2.3 +.......+99.100
3A= 1.2.3+2.3.4+3.4.3 +......+ 99.100.3
3A= 1.2. (3 - 0) + 2.3.(4 - 1) +3.4. (5 - 2)....... . 99.100. (101 - 98)
3A = (1.2.3 + 2.3.4 + 3.4.5 +...... + 99.100.101) - (0.1.2 + 1.2.3 + 2.3.4 +.......+ 98.99.100)
3A = 99.100.101 - 0.1.2
3A = 999900 - 0
3A= 999900
A= 999900 : 3
A = 333300
3A=1×2×3+2×3×3+3×4×3+...+99×100×3
3A=1×2×(3-0)+2×3×(4-1)+3×4×(5-2)+...+99×100×(101-98)
3A=1×2×3 + 2×3×4 - 2×3×1 + 3×4×5 - 3×4×2 +...+ 99×100×101 - 99×100×98
3A=99×100×101
=>A=99×100×101/3=333300
Tk mình nha bn !
A=1.2+2.3+3.4+...+99.100
3A=1×2×3+2×3×3+3×4×3+...+99×100×3
3A=1×2×(3-0)+2×3×(4-1)+3×4×(5-2)+...+99×100×(101-98)
3A=1×2×3 + 2×3×4 - 2×3×1 + 3×4×5 - 3×4×2 +...+ 99×100×101 - 99×100×98
3A=99×100×101
=>A=99×100×101/3=333300