Question

[Nguyễn Trần Minh Thư]

Tính tổng:

S=1.2+2.3+3.4+...+99.100

Answer 1

[✓ ℍɠŞ_ŦƦùM $₦G ✓]

S=1.2+2.3+3.4+...+99.100

=>3S=1.2.3+2.3.3+3.4.3+...+99.100.3

=1.2.3+2.3.(4-1)+3.4(5-2)+...+99.100(101-98)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+....+99.100.101-98.99.100

=99.100.101=999900

=>S=999900:3=333300

Vậy S=333300

Answer 2

[Selina]

Đặt S= 1.2 + 2.3 + 3.4 + ...+ 99.100
3S = 1.2.3+2.3.3+3.4.3+...+98.99.3+99.100.3
3S= 1.2.3+2.3(4-1)+3.4(5-2)+...+98.99(100-97)+99.100(101-98)
3S= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...-97.98.99+99.100.101-98.99.100
3S = 99.100.101 3S = 3.33.100.101
S=33.100.101= 333300

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