Question

Tính C=1.2+2.3+3.4+......+99.100

Answer 1

quá dễ

C=1.2+2.3+3.4+...+99.100

=>3C=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

=>C=333300

vậy C=333300

Answer 2

C=1.2+2.3+3.4+...+99.100

3C=1.2.3+2.3.(4-1)+3.4.(5-2)+...+99.100(101-98)

3C=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+99.100.101-98.99.100

3C=99.100.101

C=333300