Question

A=1.2+2.3+3.4+...+99.100

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Answer 1

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

Answer 2

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

Answer 3

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 !

Answer 4

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