123+123=123x2=246

Đặt A=1/3x1 + 1/3x5 + ......+ 1/95x97 + 1/97x99

\(2A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{97}-\frac{1}{99}\)

\(2A=1-\frac{1}{99}\)

\(A=\frac{98}{99}:2\)

\(A=\frac{49}{99}\)

1+2+3+4+5+6+7+8+9+10

=(1+10)+(2+9)+(3+8)+(4+7)+(5+6)+10

=11+11+11+11+11+10

=65